I have a DSA 58 carbine .308 cal loaded with a full 20 round magazine in my safe. It was in there for a year and I took it to the range to cycle my ammo. I never had a problem expending the ammo and all 20 rounds fired. The springs do not weaken if you have a quality mag. The same with my semi-autos. I load my Springfield XD with the full 12 rounds and one in the chamber and never had a problem. Use the magazine to its full capacity.
Nick