A shim set is only needed if backing the MD against a hard shoulder, like the barrel face and it needs to stop in a certain position (its index). Since Its nearly impossible to match the MD and barrel threads perfectly to get a perfect MD position (indexing), shims are used to create the stop point for the right index. Then a thread lock will be needed.
A hard backing is often necessary for MDs that are mounts for other devices like cans.
Jam Nuts don't require shims. If using a jam nut, screw the JN all the way back, the MD back to the JN, rotate the MD forward to the index point, tighten the JN up against the MD. Thread lock is optional, but still a good idea since a JN is more likely to work loose than a crush washer.
Using a crush washer, the CW has enough tension on the MD to not need thread lock. A CW will usually have enough compression to allow a half turn of the MD against it for indexing. If not, use shims behind the CW to space it forward. But if you have to do that, a JN is better. CWs are very popular since they do two jobs, but they aren't reusable.
Timing for accuracy is a whole different matter, but I highly doubt timing will make any difference with these rifles. I would like to see someone prove it does.
