If you use them regularly (I probably use mine at least once every 3 days or so)...it doesn't really matter. So I'll leave them fully filled, partially filled, almost empty or empty. They've been solid for years now...but that's with regular use.
If you're going to put them away for a couple of weeks between games...fill them up completely and fire off 3-5 "dry" shots (where there's no BBs in the mag) in a safe direction (i.e. behave as if a BB is going to shoot out).
If you're oiling your mags/gas, that will do two important things:
1. It will ensure that there isn't a BB stuck in the chamber that you'll forget about
2. It will blow a bit of oil up into the workings of the pistol, lubing the seals/o-rings in the slide (they dry out/harden too).
After "taking the edge" off of the gas pressure in the mag...store them somewhere safe. Not around anything flammable, not in direct sunlight (it'll raise gas pressure), not where anyone else is going to mess with them (or where you're going to loose them and be running around in a panic getting ready for the next game).
Never put BBs into the mag until you're ready to shoot. If you need to stop for a while before you've emptied the mag and can't shoot anymore....simply rack the slide manually a bunch of times until it's empty...it'll knock all the BBs out. Then dry fire 3-5 times (to clear the chamber)...and put it away somewhere safe.
Have fun, wear eye protection!