As I understand it, the consensus is that repeated compression and decompression of the spring is what causes it to lose tension. There are more than a few stories of pistol magazines kept fully loaded for decades and functioning without a problem when eventually used.
I've kept the seven shot magazine of my Mossberg 500 fully loaded for four years, except for practice and cleaning. When I practice, I usually only load two or three shells at a time, so the spring is pretty well extended and exerting the least amount of force on the shells. I've never had a misfeed, or any other malfunction in over 1000 rounds fired in this way.