i get into some pretty gnarly hills up here, and I MUCH prefer going up them then down them. I'm riding 46x16, which I think is pretty decent. I can make it up some pretty decent stuff by just standing up and digging in for a bit, and downhill I get some decent speed before the bike outspins me and I have to hit the brake to keep her steady. If you're going to be dealing with hills that outspin you frequently you're going to want a brake.

*edit

Also, to answer the OPs question, you can't really just let your feet sit on the pedals, otherwise the pedals will pull your legs which will start you swaying from side to side. You have to keep up with the bike if you want to keep it steady.