My 870 with 6 rd. tube is loaded with 5 rds, chamber empty. It stays loaded in that condition for ever. The spring is compressed enough with 5 rds. If I load six, that compresses about 5" of spring down to about 2.5". How long before the spring weakens if fully loaded? I don't know. Call it piece of mind.
My mags for my P229 are also always downloaded by one. Will keeping them fully loaded weaken the springs? The general concensus is no, but since mine are loaded constantly, I feel a little better about it and aren't too concerned with 11rds. instead of 12.
I have to admitt, that I have some magazines over 10 years old with lots of use and when I measured the length of the springs, there's only about .5" difference from a brand new one. Doesn't seem like lots of spring compression to me.
I keep single stack pistol mags fully loaded, and AR mags downloaded by 1 or 2--but that's for mag. seating and feed reliability. That applies to AR 15, Mini-14, and M1 Carbine.
Where quality is concerned, I think the industry has pretty much mastered springs.