There's no real reason to if you're just getting up the hill, except for working a different set of muscles. Gear down and spin, if that works for you and you're not racing or training, no reason to do it differently.
Everyone's different. I see little guys hopping up all the time, others just keep a steady pace. i'll get out of the saddle if I'm running out of gas and want to keep my speed up, but only briefly. There are some who go up the whole hill standing.