As a long time roadie and recent MTB convert I tend to climb sitting down wherever possible. I understand that this may not be the best approach off-road, so I've been trying to work on climbing standing up, but without much luck. If I try and stand on a steep loose climb I just end up spinning the back wheel. If I attack the same climb sitting down in my granny gear I can spin up it just fine.
On the one hand I'm happy to say that as long as I got up the climb I won but I suspect that I'd be able to get up more stuff if I learnt to climb standing up as well.