This is, now, an old wive's tale. It may have been true a few decades ago, but metal technology has greatly improved and this is not true.That not to fully load your mag, for example if your mag can have 15 rounds, you should only carry 12-13 rounds. This is to not wear off your spring over time, especially if you don't go to the range often and leave your rounds inside the mag a long time?
The only magazine spring failures I've had is one that I somehow managed to leave loaded for 9 or 10 years in the safe. It still worked, just not very well and it got replaced. The others were all on a Walther P99 that I bought used. The gun was only 2 years old. I had no idea on the history of it, though it looked to have very little use. All 3 magazines had springs that failed to lock back the slide on empty. They fed fine and worked perfectly otherwise, so they probably aren't really "failures" per se.
I always have a fully loaded mag in the gun and my back ups are fully loaded as well. If I wanted a 10 round magazine, I'd buy one; my 12 round magazines get 12 rounds put in them. My magazines stay loaded all the time, at least for whatever guns are in current rotation. If the gun isn't carried or on HD duty, it sits empty in the safe and its magazines sit empty in the magazine box.