Supposedly, leaving GI magazines loaded to capacity will cause the feed lips to bend over time. Lots of internet "experts" have confirmed this. I havent experienced it myself, but the longest I have kept an AR magazine fully loaded was about six months. No failures to feed, no visual difference from a new magazine.
As the sheep dog pointed out, Magpuls have a cover that eliminates the problem entirely. I have not personally used a Magpul magazine, but I have heard nothing but good things.