Leaving magazines loaded does not fatigue the springs. Being cycled fatigues the springs. It is perfectly OK to leave magazines loaded.

HOWEVER there is one caveat when you are dealing with thin-bodied aluminum AR15/M16 magazines. Leaving those loaded for long periods of time can cause the feed lips to slowly start to spread. This of course results in an increased chance of double feeds.

All that being said, I don't think the political situation is so precarious that you need to keep loaded AR mags at the ready. This isn't some volatile place like downtown Mosul or anything.