It's a fairly little known and rarely adopted practice to store your mags charged when not using them.
From our experience in airsoft we practice emptying our mags after use for logevity. However, if you do this with your gas mags you could be harming and reducing their life and reliability.
When a mag is empty there is no pressure on the O-rings and when rubber just sits it oxidizes, begins to break down, and will crack.
When you charge your mags it causes the seals to expand and since the gas we use contains oil the seals are lubricated as well. So if you store your mags charged with gas you will see longer use and less leaky mags.