From what I understand, a compressed spring will not necessarily wear fast. The compression and release is what wears them. I've also read that a hi cap mag will last longer with one round removed. http://www.thehighroad.org/archive/index.php/t-204014.html
I keep my mags loaded for work and keep them in my magazine holster for weeks at a time. The only time they'll get relief is if we have a training range or I think about it and I will unload them for my off days. But I've always been told to unload a mag fully to give them some relief.
I have come to learn that rounds in a magazine and being moved in and out, chambered or moved around can impact some rounds to the point of the bullet being seated a bit lower than spec. I would be more concern with this then the spring. I just learned this and not sure to what extent this can happen but I think it's one more thing to keep an eye on. I usually rotate my mags, and now will check to make sure I haven't caused any rounds to sit lower than they should.
They are designed to be loaded and ready for the fight.......Load'm up.
I do load 28 instead of 30. Not really for the spring, more for me, when you go to "seat" a full 30 round mag on a closed bolt, you need to push pretty hard at a bad angle, so, I leave to out to get a little more spring travel.....Good Luck
I have seen this question come up from time to time. The agreed to answer every time is that a spring will loose it's elasticity from being exercised not from being kept either extended nor compressed. The movement from going from one to the other frequently will wear them out. If you use them alot it is a good idea to replace the mag springs from time to time.
A major spring manufacturer says that a 1911 magazine spring will last longer if it is kept loaded light one round than if it is kept fully loaded all the time
no problem leaving it loaded. Springs are meant to be compressed and decompressed. Its no different than constant reloading, the spring is going to get worked either way. In my opinion, keep it loaded!
While it's not true, even if it were it would be a small price to pay to have to occasionally replace the springs as opposed to having your guns unloaded.