Head assembly: clearance checking importance and procedure

[Onizuka]

This is a subject I know very little about but have recently been pondering. What exatly is the method of clearence checking and what are the different types of tools you can use to do it?

Specifically to do with the SR rockers and shims: Do you measure the the ends of the rockers and buy/match to the best fit guide-shims within factory specifications? What are some negative results that can arrise from ignoring clearance checking?

To be honest, I just tossed my shims in by guessing because the box they were in got moved by a family member and shims got mixed. When time came to get things on the road I simply wanted to get things done. Since then, I have taken trips to 7500rpm and more without any problems. With all the various noises coming from various loose interior peices, noisy injectors, squeeky coilovers, clunking steering rack, rattling timeing chain, full exaust and what not, I simply cannot hear if anything IS wrong.