Yes, loaded. The only time a spring wears is when it is cycled.
I hear this claim, and I think it is BS. No personal offense intended; I think my opinion on the topic is generally the dissenting one.
A spring held under compression wears. The free height and/or spring rate will decrease. In the case of mag springs, I think it's just not enough to worry about. Just as a guess, I's say the rate ('springyness') stays about the same, but the free-standing height decreases. The latter part doesn't really matter much since the uncompressed spring will remain longer than the space it normally occupies.
As a purely anecdotal thing: I play (well, used to play, haven't recently) a lot of airsoft, and carried piles of the 'medium capacity' magazines on me. They worked just like normal firearm box magazines, but with much longer, weaker springs. Using the mags never caused any appreciable issue. Leaving them loaded between games (commonly a month or more; once even during a whole 'off-season') totally boned them. The good mags would just fail to feed the last few BB's. The cheaper ones would just not want to work at all.
Obviously, airsoft != firearms, but I think the spring thing is relatable. The real steel stuff might be so much stronger that it never causes an issue...but I think saying that loaded mags never lose spring tension, and than loading/unloading is much harder on them, is a crock.
edit: I googled it out of curiosity, to see if anyone had ever actually scientifically tested this oft-debated subject. I've yet to find that, but I did see some random dude's explanation that seems to make sense to me:
"I'm an engineering major, and we've covered this stuff pretty extensively in many of my classes. Fatigue (from the strength of materials sense) is based on cyclic loading, and is a function of the amplitude of the stress, the mean stress, and the number of cycles. I haven't run the calculations (finals are finally over, and I think my brain would revolt if I broke out another textbook), but I'm pretty sure the number of cycles to failure for a 1911 magazine is pretty high.
The phenomena you guys are talking about is called creep, where a material slowly loses strength while elastically deformed. Elastic deformation is when a material is under enough stress to deform it, but when the stress is removed the material returns to it's original shape. Springs compressed within their limits are elastically deformed. Creep is mostly a function of temperature, and typically isn't an issue in environments below half the melting point of the stressed material.
So under the typical conditions found in a house, your magazines will be fine."
If Mr. 'Engineering Major' is correct (...I'm an engineering dropout), they are simply different kinds of stress that can both be called 'wear.'