I dive with a 19cf pony slung whenever I do not have enough faith in my buddy to be RIGHT THERE. That is typically whenever I am spearfishing.
I decided to do this not after an "oh no" incident, but rather after lots of contemplation at 110' about "what if my reg dies now" as I was lining up, spearing, or stringing a fish.
I turn it on at the surface before entry, then turn it OFF prior to going in. After clearing the surface and just before starting the real "decent" phase of the dive I look to see if I lost any significant amount of pressure in the SPG (sometimes I do get a freeflow on entry for a second or so); if the gauge is still showing significant pressure I leave it alone.
You'll get one "pull" from the reg before you turn it on, which is enough to calm the instant "oh oh" and allow you to crank on the valve. Pressurizing ahead of time keeps water from accidentally entering the LP hose and winding up in the first, which can happen if you accidentally bump the purge and there is no pressure in the system. Since its "right in front of you" slung in this fashion there is never a question as to where it is or how to reach it - the reg, valve and gauge are right there where you can see them, and the bottom of the tank swings nicely up under your arm and out of the way.