The consensus here, seems to be that keeping them loaded fully does not cause a problem. Keeping them clean after so many rounds is important.
I keep my 9mm mags loaded fully all the time. I have noticed no difference in function. Of course, I have only had mine since last October.
On duty I'm required to keep all my mags "topped off". When I carry off duty it depends on what I'm carrying. If the mag has a capacity of more than 13 rounds, I'll leave one round out. Under 13 rounds and they stay filled to the top. That being said, I've never had an issue with any of my fully loaded magazines in 23 years.
Leave em loaded and full.
The only thing that wears out the spring is flexing it too often, otherwise whether the spring is extended or compressed really does not matter.
Whatever you do, don't oil them up.
I keep the mag in the gun and my 2 extras fully loaded with +1 in the chamber. They were made to hold that many, and i havent had any problems out of them except with hornaday JHP thats why I personally dont use them the first round would get hung up on the mag. Federal, Corbon, and DPX (JHP for all of those) never had that problem with them. But I would tell do what you feel comfortable doing, if you want to carry a few rounds less go ahead if you want to stay fully loaded by all means do which eva makes you happy.
+1 on the fully loaded mags.
I have always kept all of my mags fully loaded. From .22 to .45 acp they are always loaded and always will be. I have never had any problems with amy of my mags for over 20+ years. Doesnt matter what Mfg they all work just like they did when I got them.
Had an old family friend that had a 1911 mags they still carried, had ammo from Viet Nam...25 years later.
Both of my MIL PROs (PT111 & PT140) I have four magazines each. I keep two mags fully loaded (one in the gun & a spare with the gun), the other two I keep empty for when I shoot out back on my pistol range. Once ever 3 or 4 months I switch the full mags for the empty mags. Just my 2 cents.