Both you guys are right on the money. IMHO, I don't re-use shims because I think the follower is hammering the same spot of the shim 36,000,002 times between services, that tightens valve train clearances. Add some heat, extra miles, poor oil, and its a bad thing. Your engine won't like 0 and you'll know it.

Personally I haven't seen a shim worn down out of one of my bikes, and think it's ok to re-use them even though I don't. My anal theory is the follower is banging a divot in the middle, mushrooming it, work hardening it, softening it, whatever, its been beat and definitely hard to measure so it falls into the 'why not' category. I have a cheap basket full of new ones so.... "why not"? YMMV

I know it's dumb and overkill.