Originally Posted by Blacklash
Generally speaking if your dealing with threaded pins or screws, until they are snug or turn no more. I do mine one full turn each, alternating. Some do a half turn each. For example if you had 4 threaded pins you'd go one turn on each below until they were snug; A B C D.
The idea behind the above is help ensure even pressure.
I've never encountered a cpu mounting system that allows you to over-tighten it. Usually there's either a spring bar that you bend to clip it in place, or the four bolt design (which seems to be more common nowadays). In the four bolt design just tighten the bolts until they're tight, and follow that pattern suggested above.