I suppose all this mostly depends on what's comfortable for your own viewing angle. There's some obvious things to avoid like getting too far off axis on some rear projector models. (like say trying to mount a big DLP much higher or notice than your eyes)
This is something I've noticed more recently on my DLP as we end up with more games where people stand up (like Guitar Hero, Rock Band or the Wii balance board stuff.) Everything looks great when you're sitting or far enough back, but if you're too close and standing up you start to lose your picture quality at that angle.
I think when I redo whatever my next space is, I'll either figure out how to make an adjustable stand so we can bump it up temporarily when those games are in use or hook up those games in another room where I can mount a screen higher.
