There are a lot of theories and opinions about 'magazine fatigue.' It's safe to say that it really depends on the magazine and how you're storing them. I'm no expert on the subject and I'm only sharing my opinion here...
First of all, the most stressing fatigue on a magazine comes from loading and unloading. The compression and expansion of the spring causes wear more quickly. This seems common sense as anyone who has flexed a rigid material learns that you introduce fatigue at the metal joints. After the Vietnam War there were numerous stories about underloading magazines (18 rounds in a 20 round magazine). I heard many of these stories in the Army in the late 70's. They were pretty much debunked by the early 80's. The problem with magazines stemmed from overloading a magazine. Most 5.56 ammunition in the early Vietnam War era came loose in ammo cans. Soldiers frequently loaded their magazines until full. The problem was that you can fit 21 rounds in a 20 round magazine. This loading fully compressed the spring and led to jams. Many soldiers were trained to underload their magazines (18 rounds in a 20 round magazine). The jamming problems due to overloading were solved when the US began packaging ammunition on stripper clips to ensure that soliders loaded the proper quantity of rounds in the shortest possible time.
Lastly, keeping magazines loaded is not a problem for the spring. The biggest problem with loaded magazines is spreading of the upper lips. PMag plastic magazines with top covers solve most of this problem, but I'm skeptical that it really does anything other than make you feel better. A full load on a spring will not cause you problems unless you have bad magazines, or you overload your magazine.
In my opinion it comes down to two choices... you keep you magazines loaded to capacity, or you store them empty. Anything in- between invites problems.
