As an engineer I took part in some testing on springs about 10 years ago, we found that it was mainly compressing and releasing the springs quickly which took the most out of them (the spring strength reduces the more you do this, I believe it is a technique that can be used to reduce FPS via the main-spring?, though i've never done it myself).
Keeping springs compressed does damage them over time, but not enough to make a real difference.
Personally I wouldn't worry about keeping mags loaded.
Main thing I would say is keep the spring and it's channel nicely lubed, lack of lube in something like this will f*ck it faster than anything else.