Valve Spring Installed Height Variation

Kern Dog said
How did I get lucky all these years and avert destruction since I have never done this before on any engine?
Do you need to push it past 6000 rpms a lot to really risk damage?
I just find it quite curious that I've never broken anything unless the reason is that there is enough tolerance built in OR that I didn't push the engines hard enough to reveal any weaknesses.

You probably never had anything that really needed to get that specific. The factory didn't use spring shims that I know of. I've torn down boocoos of original engines and never seen them. But then, also the factory used mild cams with mild spring pressures (for the most part), so there was really no need. Nobody said you have to do it.