Proper posture is all well and good, and in fact, I nearly always use a psuedo-classical position when sitting. Standing up, though, is a killer.
Proper posture is virtually impossible then.
I've done a lot of sitting down on stage, but the stage presence just isn't the same. We must remember that the angle and style of play will make a BIG difference in causing or alleviating pain problems. The 4 string F chord is a good example of this, check it out. Many other chords don't affect the wrist this way.