There are still mags made with stainless steel springs that will take a 'set'.
This is a myth that was widely believed for many years, pretty much discounted for a while, and has now resurfaced. It's still a myth.
That being said, leaving a magazine loaded for extended periods, particularly if the magazine is actively carried (as in LE work), can cause some deformation to the cartridge cases themselves. If this process goes far enough, failures to feed may result from the misshapen cartridges. Some who experience this may blame the spring, but they're barking up the wrong tree. Prevention is as simple as emptying the magazine and reloading it from time to time, even with the same cartridges. This works just fine as long as the cartridges are not loaded in exactly the same position in the magazine each time.
My personal preference is to prevent loaded magazine storage issues by emptying all carry magazines through a full firing cycle at least a couple of times a year just to make sure everything works as designed. It's also a lot more fun than unstuffing and restuffing the magazines.
Abuse of the magazine spring such as stretching it to "improve feeding" is another issue, and will permanently damage a spring. The only way to restore reliability is to replace it.
I was personally more than a little dismayed to hear from a Marine recently back from Iraq that the troops in his unit were
directed by their gunnies to disassemble their Beretta magazines and stretch the springs
daily to "maintain feeding reliability" and prevent the failure to feed incidents they were experiencing. Apparently no one told them that the practice itself was the most obvious cause of the issues they encountered, and the risk was increased each time a spring was stretched.
A fairly detailed explanation of the science around the myth of pistol magazine spring set, as well as several firearm manufacturers' validation that spring set is not an issue, can be found in a May-June 2003 article from American Handgunner Magazine. The full text is available at:
http://findarticles.com/p/articles/mi_m ... i_99130369
