Peter, PCP's are designed to be left charged, I keep mine filled all the time. I shoot frequently, so I top up my guns after a session so they're ready for either the next session, or marauding starlings and sparrows :-). If I was going to put one in storage, I'd drop the pressure to maybe 1000 psi, but I wouldn't empty the tank. The only reason you should need to drain the air is if you need to work on the tank, or you're transporting your rifle by air. FAA regs require pressure vessels be empty. Hope that helps.
Dave