I usually stand for very short climbs where I don't want to have to shift down or lose any momentum or near the top of a climb to get a little kick up over. Sometimes I'll stand for a little bit on a long climb just to stretch out the legs a bit or again to get a little kick up in speed. Really though it's personal preference. Some people climb best standing, but for most of us seated climbing is a bit easier on the cardiovascular system and leg muscles.