Do you cycle your home/personal defense ammo?

[TFOGGER]

If being compressed ruined springs, car springs wouldn't last 25 years at a time.

I guess I just haven't had any of my guns long enough to ruin them by leaving the mags loaded. Mine have been loaded for going on three years now, and have yet to have a failure related to mags.

Ever look at the suspension on 70's Chrysler products, where you'd have to put 4 inch extended shackles on 'em just to get them to sit level(in the early 80's)?

Springs will take a set, and lose tension when placed under stress. This effect is less noticeable in helically wound springs (most magazine springs), and those that are lighter in rate, but longer and with greater preload.

[ColoEnthusiast]

Ever look at the suspension on 70's Chrysler products, where you'd have to put 4 inch extended shackles on 'em just to get them to sit level(in the early 80's)?

As foxtrot mentioned earlier, there is a difference in types of springs, with helical being better than leaf springs for durability. I have also seen very old magazines that are fine and have been sitting loaded for over 40 years (M1 carbine primarily but others as well).
From what I have read, spring metallurgy will hold up very well under constant (but not excessive) pressure, but use is still the best way to wear out a spring. As for the old cars, there are many people who park uphill and leave excessive stress on the rear springs. That along with 200k or so and/or towing will take their toll. Another possible factor may be that some cars were simply made with higher quality spring steel...
I personally leave out a few rounds from a magazine and don't worry about it. I still rotate magazines on occasion as well, but not because it is a big concern, more for peace of mind than anything else.