I'll start a hill in my 39 X 17 and sit and pedal as smooth as I can. If the grade goes up/I get tired, I'll switch to the 39 x 19 still sitting, and that is the lowest gear I will let myself use unless it is one of three brutal hills that are available on the different routes I do. Then I will allow the ***** 39 x 21.
Since most of the hills I do are a mile or so or more, I'll stand at some of the slightly steeper sections to keep my rhythm going, and sometimes do over half a hill climbing.
This comes from being a runner, and just being comfortable standing, and when I started riding originally, I was in southern Germany, where I would have to ride hills that were a few miles long every other hill. Standing just felt more natural.