Using them wears the spring and to a lesser extent, everything else.
Leaving them loaded doesn't hurt anything. I've left mags loaded for years and never had one issue when I used them after.
If your worried about it, leave one less round in your mags or rotate them on a regular basis. I don't see any need to do that though.
When we do hit it we hit it out of the park and send it over to China as quickly as possible to cheapen it and sell it.