I use my pistol mags a lot (at least once weekly...usually every couple of days because I'm building something or other)....so I leave them however they are when I take them out of the pistol (empty of bbs though). Fully gas, partially empty, full empty, etc....doesn't matter. They've worked fine for years. They're failing from hard use...not from wearing out.
But, if you're going to use them once a month....or less than frequently, do the following.
1. Make sure your gas is oiled
a. put a drop or three onto the fill valve of each mag just before you gas it
b. oil the whole propane tank (I've got a special do-hickey that lets me inject a bunch of silicone oil right into the tank....never have to oil each mag separately again...just the tank once when I first use it...I go through a lot of gas though...I have 8 empties kicking around and 6 fresh cans ready to go...and that's just from Jan until now)
2. Fill your mag completely with gas...no BBs
3. Point your pistol in a safe direction and rip off a several shots. This will clear your chamber (you should still eyeball it to see that it's clear though)...and it will blow oil up into/around the loading nozzle, hopup rubber, piston/seal, etc...
4. Shoot until most of the mag is empty (i.e. if you normally get 30 shots...shoot 20-25). Then put it away.
Normally at a game you have rounds still left in your pistol...fire those off and put it away. Unless your mag runs empty of gas...there'll be enough to keep a little pressure (but not a full charge) on the seals.
Use your common sense...you don't want the seals to dry out (but if they do, a good soak of oil might bring them back to life).