Constant tension on a spring does not weaken the mag spring. It's the emptying/refilling cycle of the mag over time that does.
I've always kept every mag I own fully charged when being stored. Admittedly I am a casual shooter and don't shoot multiple 100's of rounds when I do go to the range. And come to think of it, I've never had a magazine spring break on me due to use or wear.
Just my $.02 worth.
