Both. I like going fast on the downhills, but I still shift to the highest gear ratio that the bike has and I pedal.

As Sheldon Brown says about fixed gears: "Coasting breaks up your rhythm and allows your legs to stiffen up. Keeping your legs in motion keeps the muscles supple, and promotes good circulation."

http://sheldonbrown.com/fixed.html

Same idea here, it's just that I put a lot more effort into the climb, since gravity works against you there.