Metallurgical theory: a spring's performance will deteriorate only with loading and unloading. A perfect spring will not lose performance over time just because it is in a compressed state -- assuming you do not over-compress the spring. It is the "working" of the metal involved in loading and unloading that will affect performance.
Real life: no such thing as a "perfect" spring. As general rule of thumb, load to -10% capacity. So for your 7 round magazine, I'd say load to 6 shells.
The above was relayed to me at Insights General Defensive Rifle. I'm sure other schools may teach differently. An interesting tidbit of info was that Greg Hamilton, chief instructor of Insights, keeps each and every magazine he owns loaded (to 90% of capacity). In his opinion, an unloaded magazine is like an unloaded gun -- a paperweight.
HTH,
Justin
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Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania
