I think you'll find a lot of variation on this depending on when and where people ride. For me it is one serious headlight in front and a standard rear taillight. I experimented with Tireflys on the wheels -- particularly on the front since I think it would increase visibility at road crossings. Unfortunately, I found them unreliable and annoying to deal with since there was no easy way to turn them on and off.
If I rode more on busy streets or highways I would certainly want more lights from the rear and from the sides.