I'm a total sitter. I'll stand a few times a year, on a very steep long hill when I just feel like I need to change something or fall off the bike.

It depends on the bike, too, though. I think if I had a different bike I *might* be more inclined to stand.

However, my weight distribution is such taht the one time I went skiing, if I fell down I could not get up. My backside has little pockets of extremely dense tissue in it or something

Any slight differences in efficiency really aren't that important to me and would be counteracted by the necessary change in focus... and besides, I live in the flatlands so I only do hills on special occasions.