I heard somewhere (I don't remember where) once that a seat post clamp should be oriented with the gap at the front of the seat tube, opposite of the gap in the top of the seat tube. The logic behind this was that it supposedly exerts more equal pressure around the entire seatpost.
However, I've noticed that bikes off the rack are almost always sold the other way around, with the two gaps on top of each other.
Does it really matter? Is one way better than the other?