My advice: don't fret over there being an ideal storage pressure. The proper amount is some. Some pressure to keep the seals formed against the extrusion gap.
Granted it is true that higher pressures have an influence on the material aging (hardening and cracking) but I suspect it would require well controlled lab conditions using the same production run, same temperature and humidity, same lubrication, etc. to detect the difference.
A regulator doesn't really care provided you don't expose the gun to a massive temperature rise that dimples the valve seat.
There are some rare exceptions like a poppet slowly extruding but from my point of view that is an indication the poppet is ill suited to the task. I feel the same way regarding O-ring extrusion....if it extrudes and fails, it was an insufficient durometer.
I keep all mine full or nearly full but I see nothing wrong with putting them away with 500 - 1000psi if it suits you.