We used to keep all of the Military Police ammo in the mags for extended periods of time, never had a problem with M-16/4 Mags having a problem or the M-9 mags. And were talking ammo supply for a couple hundred folks. Every now and then it would be cycled to ensure a pencil or something wasn't exchanged for a bullet...lol
I keep all my mags full on my stuff and never had a cycle problem either, but the cheap plastic .22 types mags, not sure about.