Generally, full capacity factory mags and 1st and 2nd-World military mags can be fully loaded without any significant spring wear.

Aftermarket mags, mags from 3rd-World militaries, and [b]Klinton-reduced 10-round mags[/b] will usually have spring-set problems if left fully loaded.  In the case of the former, it's because the materials used are cheap and poorly engineered.  With the 10-round mags, the problem is the BATF regs which force manufacturers to use poor designs to meet BATF approval.

-Troy