General rule of thumb (no pun intended) is thumbscrew mounts do not return to zero.
That said, if it's a well made mount and you mount is consistently every time, it should (or atleast be very close).
Always push the mount forward in it's slot as you tighten the thumbscrews. Tighten the thumbscrews in the same order every time (front then back or vise-versa). Tighten them the same amount every time. I usually tighten them by hand tight and then use a tool (coin, screwdriver, whatever) to tighten them another 1/4-1/2 (depends on thread size, sometimes 1/4 turn doesn't do much) turn. If your thumbscrews are tight at 1/4 turn extra, always tighten them 1/4 more. Same with 1/2 turn, just don't tighten them 1/4 turn one time and 1/2 another. Consistency is the key.