|Okay... another long one, so pardon the abuse of banndwidth.
Using or not using washers with Thackeray Washers (springs) is not the make-it or break-it deal this thread seems to be building it up as. Yes, it would be preferrable to have a washer above and below the Thackeray Washer, but it does't fail to work if one or both are missing. I'll get back to that in item 3), below.
1) Job #1 is to get the soft mount gaps right.
Z-S carbs use one soft mount O-ring, and the gap between the two metal surfaces sandwiching the O-ring should be a uniform 0.070" (1.78 mm) all around the joint.
For Dellortos, the factory original soft mounts use two O-rings along with a die-cast sandwich plate. In that case, the gaps on either side of the sandwich plate should be 0.040" (1.0 mm). More specifically, 0.35 - 0.50 inch (0.89 – 1.27 mm).... but 0.040" (1.0 mm) is a good gap for the J-H/ Lotus die cast soft mount.
The 1980-onward Lotus soft mount for the 910 Turbo is different, it uses much fatter O-rings, and the specified gap is 0.080" (2 mm) per side. Know what you have, and use the correct gap spec.
There are many brands of soft mounts out there, but there’s no standard thickness of O-ring, and hence, no standard gap. O-ring and gap sizes vary, so read the instructions that came with yours, and use the correct gaps for it.
If your soft-mount spacer plate is plastic, then understand that plastic continues to shrink with age. Slowly, but persistently. The spacing between the bolt holes in an old plastic spacer may be shorter than required, causing the plate to bind on the studs and not slide freely. The bind can be tight enough to interfere with the spacer's ability to freely seek a centered position between the two flanges on either side of it (carb & manifold), frustrating your attempts to achieve uniform gaps. Don't use a soft-mount plate that will not slide freely.
A newer type of soft mount consists of a thin, stamped metal plate with an integrally molded rubber "O-ring" around the center bore (throat). It's a one piece part instead of the O-ring / plate / O-ring assembly. It's easier to handle, but thinner overall than the standard sandwich plate. I’ve also seen non-metallic versions of the same theme. Those thin-plate mounts typically have less total rubber for compliance, and don't follow vintage assembly conventions. If you use them, ignore the Lotus gap specifications, and follow the instructions that come with those soft mounts.
It's more important that the gaps be uniform on both sides of the spacer, all around the spacer, and spacer to spacer, than it is that the gap be dead-nuts one specific value. First be within the broad spec range, then be uniform in the use of whatever value you use.
2) Job #2 is to get the compression of the Thackeray Washer or rubber grommet correct. That translates to the amount of clamp pressure applied to the joint.
If Thackeray washers (springs) are used, then limit nut adjustment to maintain a minimum gap between the spring coils of 0.035" for the final setting. The full spec range is 0.035–0.050 inch (0.89–1.27 mm), but you’ll often find that the "minimum" coil gap (tighter) is required in order to maintain the correct gap on either side of the soft mount’s sandwich plate. Make sure the coil gap is the same for all the Thackeray springs used (ie, all studs & nuts are equally tight).
If rubber grommets are used, then tighten the nuts until all clearance/ slack is taken up between the nut, cup washer and rubber grommet (ie, the stack-up just goes solid), then tighten the nut another 1 1/2 turns, ensuring that the "V-grove" in all grommets is equal. Since the M8 nuts have a 1.25 pitch thread, 1 1/2 turns is equivalent to 1.88mm (0.074") of crush/ pre-load. If more clamp force is required to get the soft-mount gaps in spec (above), then 2.54mm (0.10") of compression is acceptable, but don't over-tighten and crush the grommet. Keep the setting uniform for all mounting nuts; and ensure that the "V-groove" in all grommets is equal.
It may not be possible to achieve both a proper soft-mount spacer gap and a proper Thackeray gap / grommet compression at the same time. If there's a conflict, then the spacer gaps are a direct reflection of O-ring compression, and take priority over the Thackeray/ grommet compression.
3) Job #3 is to make certain that the stud's threads stick out beyond the Nyloc nut by at least two full threads... three if you're not certain what constitutes "two full threads". If you see less exposed thread than that, then omitting the inner washer between a Thackeray washer and the carb flange would be more acceptable than proceeding without adequate thread exposure.
The ends of the Thackeray washers are closed & ground (ie, flat), but they're not solid. As the spring flexes, the ends can move/ squirm. For that reason, it is best to have a washer on either side. That's 'best'. But it's not a case where the Thackeray washer fails to do it's job if no washers are used.
If there's not enough thread protruding beyond the Nyloc nut, the cause is probably due to the stud being too short, or a tall 'full height' Nyloc nut being used. It would be best to fix the problem at it's source. However, if 'saving your day' is a simply a matter of removing the inner washer between the Thackeray washer and carb flange, then no big deal.
But that's where "I" draw the line. My paranoia doesn't like the idea of the end of a squirming spring working directly against the nut that is supposed to secure it. Especially on something as important at the carbs. I always use a washer between the Nyloc nut and the Thackeray washer... and I recommend that you do as well.
So, in the three Job priorities above, the washers over/under the Thackeray springs are something like Job #3.5... or 4. They're not the first thing to worry about, or to allow to derail your project.
Thackeray washers experience metal fatigue due to the vibration, and fracture. The upper ones are more prone to failure, while the lower ones tend to "last forever".
The rubber grommets are fuel resistant, but not fuel proof; so fuel leakage (or carb cleaner/ brake cleaner) will eventually attack them. Since stuff runs down hill, the lower grommets tend to fail first, while the upper ones see little fuel and “last forever”.
A compromise solution for long service life is to use Thackeray washers on the bottom and grommets on the top. However, when you mix Thackerays and grommets, the clamping force may (most likely will) vary between the two types, so pay even more attention to keeping the gaps on either side of the soft mount sandwich plate uniform. That’s still Job #1.
Thackerays will hold their setting very well over time, however the same can't be said for the rubber grommets. Any rubber or polymer elastomer will take a compression set, yielding to an applied load over time (it's called Creep). If you use grommets, then re-check the fittings & gaps periodically. Especially re-visit them shortly after a new installation, since new grommets will quickly take an initial compression set.
Off topic, but the same Creep issue applies to the inlet needles with rubber tips. The rubber will take an intial compression set, changing the float height you so carefully set. After installing new rubber-tipped needles, go back in after 2-4 weeks and check/ re-set the float height. Solid brass needles don't need to be re-checked.