FWIW, I mount it valve down, so under water the valve is right at my hip and I can easily turn it on or off during the dive. It can also be removed and handed off if needed without too much trouble. But so far I've never had to use it.
I too mount mine upside down, but more because its easier to reach back to trace the hose routing if my reg comes off its bungie. I can't do that if its rightside up like I can do with my main tank. I get a lot of strange looks on the boats I've been on tho, like I was putting my fins on my head or something.