if you want to read about springs, loads, and failures see
http://www.navsea.navy.mil/nswc/card...APTER4RevD.pdf
See sections 4.3 and 4.4.1
I interpret this as saying that keeping a magazine full can weaken the spring over time. If you wish to keep it full, keep it full but periodicly empty the tube exept for one round and push it in to feel if the tension is weak.
Remember, if you ever have to fire down from an upstairs landing the spring will be working harder than if you are shooting on a horizontal plane or up.