Horizontal (when canted) and spinning perfectly. Horizontal to NOT see it and so the BH would/should never engage my hand/finger if something went wrong. I am RH'd and I cannot bend my left index finger from a childhood accident, so that sticks out there a bit and I have to corral it in with my middle finger to keep it tucked. I don't care to have "inches" of arrow in front of my riser at full draw and like to keep that whole dynamic neat and close to the bow. I can literally draw a horizontally mounted BH over the top of my hand without risk of injury. Also nice when shooting selfbows and similar w/o a horizontal rest. G. Fred suggested this years ago(1989?) Re: Zwickey Deltas to me, and I've stuck with it ever since.
There is no right or wrong, it is personal preference and everyone has there own reasons.
I disagree with the arrow flight argument Hor. vs Vert. The arrow is spinning inches/feet from (unless no helical or off-set) the bow upon release, and will correct itself per your correct spine & tuning efforts, regardless of the initial BH orientation.
Kris