If these are hills you encounter frequently (i.e., they're on your standard training route), I think the best course of action is to try a variety of techniques until you find the one that gets you up and over the top in the most efficient/effective manner.

Also, by repeating the same hill over and over again, you will get more comfortable and learn exactly where your limits are

Realize, however, that your method of attacking the hill depends on how you feel, how many miles you've done up to this point, what your training goals are for the ride you are doing, and weather conditions, headwinds, etc.

Also, sometimes I find a small hill to be the perfect excuse to give my butt a nice break by standing!

SteveE