I'm not certain it matters, as long as the head is on straight, especially if you are using helical fletching. By way of thinking, if you have the head aligned up and down, when you release, the arrow flexes back and forth (then maybe up and down afterwards). After a few feet of travel, it is spinning. I would think that a horizontal alignment would create less possible issue at release (a big fin out front), but after it starts spinning, who knows. I have never seen a slo mo of the arrows flight after maybe three feet from the bow. Lots available at the bow itself, but none on the flight. I would be interested in seeing what an arrow actually does in its flight.
ChuckC