Never a bad idea and there are just as many theories out there as to when you should do it, as their is to how you should do it. I rotate mine every couple of months, however I probably empty them and rotate the ammo more frequently.
No point in rotating mine, they all stay loaded all the time... except for a couple of dozen AR mags. I only leave a few of the 30s loaded.
I've yet to need spring replacements in any of them but I'd rather replace springs than get caught one mag short of what I need cause I'm letting springs rest. My understanding is that the cycling of the springs is what wears them out anyway, so it would seem cycling them would serve no purpose. I try to use them all evenly at the range, that will have to suffice for cycling.
Mag springs don't really seem to care what you do to them, unless you streeetch them, in which case they protest violently and give up.
Recent studies have suggested that modern springs do all the "setting" they're EVER going to do in the first week or so of compression... and that seems to jibe with what I've experienced.
Still, I DO rotate my mags (for the guns I keep loaded) at least every month or so. I'm not sure it's really necessary; it's more force of habit.
One thing that IS recommended,though, is removing all rounds from your mags and reloading again, preferably into another mag. Sometimes, after periods of carry, the rounds will shift out of position enough to cause malfunctions. Rotating your mags eliminates this potential problem.