I sling my 19cf pony, so this is an uninformed conceptual opinion, but if I dove with a pony back-mounted, I'd dive with the valve down. It would make it easier to see bubbles coming from the wrong place if I were to log-roll while solo diving. Mounting it with the valve up would strike me as a poor decision, considering I really like the option of conserving air in the event of a mild gear problem.
That said, at the end of the sixth dive this past weekend, I noticed a tiny stream of bubbles coming from my slung pony's O-ring. There is no way I would have noticed such a small amount of bubbles, even if it was mounted upside down, which makes the "you can see failures more quickly with your tank-mounted pony inverted" idea carry somewhat less weight.
(Seeing the tiny leak while at the end of my safety stop prompted me to investigate, and when I pulled the reg after the dive, I found the O-ring was not in good shape and likely would have blown completely sometime in the next few dives, judging from the pieces starting to extrude/shear/whatever off.)
