I always leave my mags completely fill with gas.
Like some has previously stated, they keep pressure on your O-rings and your seals giving you a healthier mag.
Leaving it filled with crappy propane can leave you with a nasty guck or this weird corrosion inside the mag however. propane like coleman, rona, etc
I'd go with a much more cleaner alternative and use bernzomatic's torch propane, much more cleaner, and from what I experienced, doesn't smell like poop.