I've heard the argument before. Some say don't do it, or only keep them 3/4 full. Others say they've had full mags loaded for years.

I have one mag loaded full at home. The rest are empty. In my mind, keeping a spring compressed is fine. Using it frequently is where it *might* get weakened to some extent. Think about how a handgun *breaks in* to some extent with use.

So, maybe? Yes? No? There's debate on this issue all over the interwebz.

On pistol magazines, I load to capacity but I use these magazines regularly, cycling through the ammunition. My centerfire bolt action rifles are primarily varminters, so they're loaded one round at a time. Centerfires with detachable magazines are loaded as game laws or competition rules require...usually 5 rounds per magazine.
Springs are an often-overlooked matter. Their function is simple, but spring technology and metallurgy has noticeably improved over time. In the past, compressed magazine follower springs were noticeably affected by long periods of compression. Options have improved, and today's quality magazines utilize better springs. I rely primarily on McCormick or Wilson magazines for my .45 ACP and, after years of use, I can report no failures to feed or problems with slide stop activation that I can attribute to a weak spring. I had WWII-era .45 magazines that did exhibit spring fatigue, and those springs have since been replaced.
I would not load every magazine to capacity, only the firearms I rely upon for self-defense, and I use those in practice. It makes little sense to load a magazine to capacity that you know will sit dormant for an extended period.
Briefly, if you shoot regularly and use those magazines, I doubt you'll encounter a problem with magazine springs.

My officers kept their magazines loaded to the max all of the time and never rotated them, however we had quarterly firearms training so they fired their weapons almost every 3 months. I have left S&W magazines loaded for 4 or 5 years and haven't had any problem with them.

I cant speak for magazines but autoloading shotguns should be stored in the fired position to keep their springs from wearing out. I learned this the hard way through my own experiences buying used guns.

I store hunting rifles, shotguns and pistols with empty magazines; more a matter of safety. I store AR 10-30 round magazines loaded to one less than capacity for easier feeding of the first round (personal preference). I load up pistol mags. I've never experienced a problem with spring weakness. I store all rifles and shotguns in the fired position to save firing pin springs... probably not necessary but it sure doesn't hurt.

I never store a centerfire weapon cocked or with the action open. I load AR 30 round magazines with 20 rounds, 20 round magazines stored empty, and 10 round magazines with 10 rounds. Anything in the safe is unloaded for storage. Pistol stays loaded 1 below capacity just because 15 is good enough....

Magazines stored empty. I can see no reason to store them loaded. Not up here anyway.