OK, so most people do realize there #### won't fall off by leaving the mags loaded. The second part of the lesson as mentioned by BUM is that magazine springs are best viewed as a consumable item. Just as you will over time need to replace the recoil spring in your pistol, so to will you need to replace the mag spring.
Springs generally take all the "set" they are ever going to during the very first compression cycle. If you get a set of new springs, do yourself a favour and check for yourself. Note the springs as new are of the same length. Load one into a mag and load the mag. After a few cycles, pull it out and compare it to the still new mag spring. Odds are it will be either exactly the same length, or just a small fraction shorter.
Depending on your use, I would tend to keep one fresh spring as a guide, and swap out the mag springs when they have fatigued enough to be one full coil shorter than fresh. Depending on the material used and the use it recieves, that might be in 6 months or 60 years.
Until then, feel free to leave them loaded.
As usual, my opinion and $1.36 will get you a Timmies.