Leaving mags fully loaded can cause the springs to weaken but it's not a common problem with decent quality firearms & magazines. Before you decide you have a problem with your equipment you should probably do some checking to make sure.
Mag springs are cheap and easy to replace. Leave the mags fully loaded and check them regularly for proper function. You're already doing regular checks of your self-defense equipment anyway, right?
If you notice a problem then replace the mag springs with high-quality parts. Leave them fully loaded again and keep checking them as before. Maybe you just had a bad set of springs. If the springs weaken again from being fully loaded then it's possible that the magazine design is overstressing the springs when the mag is fully loaded.
Now you have a decision to make. You can:
...keep leaving the mags fully loaded and just replace the springs at intervals that are frequent enough to make sure the springs don't weaken enough to cause a problem before they're replaced.
-or-
...underload the mags by a round or two. It's the last bit of compression that puts the most stress on the spring.
But the odds are that you won't have a problem in the first place if you have good quality equipment.