Thinking of picking up some steel Beretta mags while they are on sale.
Anyway, seems I've read that the poly mags like to swell or crack when left loaded for a long time.

Its my opinion that long term storage of loaded magazines should only be GI. Don't care what others do but that's what I do.

PMags are good to go.

Earlier gen 1s had some issues with splitting, but that was a very small percentage.

They've had one of the first off the line loaded for over 10 years now, its still works and has not had an issue. If you're that concerned put the dust cover on which takes the pressure off the feed lips.

I agree.

At that price you can't go wrong.

What do you mean by "long period of time"?  

All polymers placed under constant tension will, over time, undergo a permanent dimensional change known as "creep".  For that reason, metal magazines, which do not experience the same molecular slippage would be a better choice for truly long-term storage (i.e. years).  You can get blackened stainless steel magazines from https://www.cdnnsports.com/ for about $12 each.

And I know as soon as I post this, a thousand P-Mag fanboys are going to come roaring in saying "It doesn't happen to P-Mags".  Well, since creep is a phenomenon inherent to polymers and P-Mags are made out of a poymer, unless that P-Mag magically transmogrifies into iron or aluminum when the cartridges are loaded into it, it does.  It doesn't happen overnight, but it does happen.

Store magazines empty and keep the ammo clipped.

Quality GI mags should be fine.

I found that PMags are fine as well after digging some out from my father's estate that had been loaded for years. They still had the covers on them but they say they aren't needed now. I put them back in storage with the covers on anyway.

Every Troy mag I pulled out had spread to the point where they would only sort-of go into one rifle with no ammo in the mag.

Lancers, even the early style were also fine.

Keep to mags that are known to be good mags and you will be fine.

Lancer has always had steel feedlips.Early style , middle style, late style. All the same.

USGI mags have now been around for 50 or so years.  Polymer probably since  the 80s.  Anyone know of an or lite or thermold left loaded since the 80s???

Yes, but the early models had MUCH less steel.

The orlites all cracked and the thermolds melted.

I leave Pmags, lancers, USGI mags all loaded up without any worries