I mount mine right side up, have it fully on.
Some people mount theirs upside down, just pressurize the hose then turn it off. I heard about an absolute CF recently, when an OOA diver ended up getting the not-really-on pony reg. Can you say "Panic"?
The only time I ever mounted upside down was on a trip where I had a reg issue that couldn't be serviced locally, a minor freeflow.
Right side up keeps the first stage out of harm's way -- it's easy to whack it if it's upside down.
Some folks like upside down, say that it prevents a freeflow from emptying the pony. My pony's reg is in a place where it's readily noticeable, and the reg is detuned sufficiently that freeflow is simply a very low likelihood. I also do check the pressure between dives.
Either way, good job on having a backup.