Should magazines remain loaded when not in use?

[Sunray]

"...13 out of 15 in the mag did not fire..." That's not caused by the mag. Cartridges that don't go bang have most likely been stored where extremes of temperatures and/or humidity cause the powder to deteriorate.
Mag springs (nor any other spring) do not lose temper from being compressed. Nor does repeated use of a mag cause the spring to wear out. Flat springs can get work hardened at the bends that way, but the spring doesn't lose temper.
There's no law about storing mags loaded either. You cannot store or transport a loaded mag in its firearm though.