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.
Diesel pickup, lift kit, 10mm, loud exhaust, big dog.... You didn't get enough attention as kid did you?