I sit however is comfortable at the time and shift positions as needed. If you just keep your upper body relaxed things tend to fall where they should without much work. If I'm doing a sustained hard effort (like in a time trial), sometimes I'll shift my hip angle "back" for a few seconds to let my quads get a brief recovery while I'm still keeping some power to the pedals by using my hamstrings a little more than usual. I find that I do the opposite on climbs- I tend to sit upright/back, but will occasionally roll forward a few seconds every now and then. The point is, though, I position myself to whatever feels "right" at the time.

Take home message- as long as you are relaxed, do whatever feels natural.