I love me some Hooke's Law! Springs wear out through use, not through long term compression. Loaded magazines have been located from Vietnam, in Vietnam, all wet and nasty, and both the ammo and mag have functioned.
Car springs, however, do wear out. Notice how all land yachts from the 70s ride around lower in the rear? Those springs are weaker than the fronts, no engine to support, and wear at a faster rate, but the fronts have lost some 'spring' as well. Recoil springs in ARs have a lifespan of 5K rounds I think, don't recall offhand. The also recommend that competitive pistol shooters replace mag springs and recoil springs and such at certain intervals. Most of us will never shoot that much.
Long story short, you can leave your mags loaded, your strikers or hammers cocked, and your slides and bolts locked open.