I've had a .177 and .22 Marauder for 8 years and the mags are always loaded and never had the first issue with them.
I've had the .25 for about 2 years, and i have had a few issues with these mags, but i attribute these issues with problems with the mags themselves, not they are kept loaded.
All three guns are shot weekly so they are continually exercised, if i were storing the guns for several months i probably wouldn't keep them loaded, but i cannot attribute any issues with my weekly shooting of all three guns and keeping the mags loaded.