Stand vs not stand is mostly a personal preference. Also it depends on the hill. Generally speaking standing provides short bursts of power. In other words you produce more power, but expand more energy. So some people do it on shorter/steeper hills/section of a climb. Also it's nice to do just to stretch your legs. For sustained climbing most people just seat, find comfortable cadence and chug along. It is also a great way to work on your pedaling technique. Concentrate on smooth "circular" motion. Clipless pedals help with that.