I guess this could turn into a "heated" debate, but I don't think it matters which direction the air is going, just make sure it's going.
If the same volume of air is moving across or over the heatsink, the heat is being transfered.
I guess someone with a little time on their hands and an accurate thermometer could experiment and report the results.
On a personal observation note, I would think that air being directed to the heatsink (blowing down) would be more efficient, becuase it "hits" the heat sink instead of the action of pulling air past the surface of the 'sink.
Let the flaming begin.