Although it's a useful method to stretch alternative muscles, overall you end up wasting more energy by being out of the saddle. Unless you're racing someone and need a spurt of speed, I'd tend to stay in the saddle.
Watch out for your SIJ, standing probably did for mine
ouchMay be good for your buttocks but not good for your goolies when the chain snaps mid climb as I can testify!
(Half way up Birdlip Hill on the outskirts of Gloucester).