Don't think about it too much - it's not really that important when or where you stand up. if you feel the need......stand up.

The general rule for when I'm singlespeeding, is I only get out of the saddle if I have to, or when I need a stretch. Part of the whole concept of single gear riding is to be able to focus on your spin, and generating power by pedalling in circles, not just stomping on the pedals.