I do alternate keeping magazines loaded, usually empty them out about once every 6-8 months. Now that being said, I bought a 1913 vintage Colt 1911 that was brought back from WWI. I bought it from the grandson who inherited it and wanted nothing to do with guns. It came with 7 rounds of 1918 stamped ammo in the magazine along with a sock filled with about 25 more rounds. He said his grandfather kept it in the cab of his tractor and he didn't remember it ever being shot. He said his grandfather brought the gun back with him from France in 1918 and used it as a "truck" gun.
I unloaded the magazine, did a bit of cleaning of it and the gun, and shot it. Functioned fine. I have bought several loaded magazines from WWII and the Korean Conflict, both 45 auto and M-1 Carbine, and aside from cleaning, there was no problems.