I read something recently that said there is no real benefit to climbing out of the saddle in terms of power, energy used, speed, whatever, but there is a psychological benefit in that it can feel better mentally to switch positions and push in a different way. The conclusion was that it's worth doing, because you'll feel better, even if there's no real reason for it.
Heart rate and oxygen uptake are higher when stood up, its just more efficient on a steeper hill as peak power is higher so you can get over the hill quicker. I've also read (and experienced) that it helps your unmentionables if they are feeling a little sore from the saddle.