As far as steel mags go, nope, and you can leave em' loaded full for a long time too, won't hurt em' a bit. The spring wears out from moving, not from sitting still (whether compressed or not).
Some plastic mags that have plastic lips may deform over time. But I've heard of steel mags being left loaded for years and still working just fine.
No. Springs are worn out by being compressed and then uncompressed over and over. In retrospect though, mags should be cycled through, cause if they are just sitting there then that means you aren't shooting.
I do wonder if one filled a steel ak mag with polymer coated steel cased ammo, given that the polymer cases will rust over time, if the ammo would rust along with the magazine and become locked up and unreliable.
I would only leave brass cased or lacquer coated ammo in my mags for long term storage, if I were counting on them for SHTF or something. I wouldn't care for range mags
If you are concerned with long term storage options of loaded mags for the Ak; I would suggest using either Golden Tiger with the sealed primers and bullets or Yugoslavian brass cased corrosive ammo.
Keeping the mags loaded will generally not hurt them. I have kept loaded steel, polymer, waffle type mags for the Ak loaded for years without issue.
As has been mentioned, mag springs are worn out by use, not by sitting. At least this is true for the last 70 years worth of mag spring steel manufacturing.
All mags are different however...
A fully loaded 30 round polymer AK mag vs. a fully loaded steel 1911 pistol mag, I'd expect no problems out of the pistol mag after long term storage but I'd expect the polymer mag to have issues from swelling. But the spring in that rifle mag would still be good!
A good rule of thumb for long-term storage of high capacity mags will be to download the mag by a few rounds to help alleviate pressure on the feed lips and the mag body. Probably good practice for polymer mags, and certainly won't hurt steel mags.
Don't have any polymer mags. Don't have any bad springs. Load 'em up and don't worry. What's the worst that could happen, an FTL or two? They're no damn good unloaded.
SOP for our unit was always to download 2 or 3 rounds from a 30 round mag. The only time I fully load 30 round mags is when I'm going to the range knowing I'll shoot them.