Don't replace mag springs unless the mag is malfunctioning. To do otherwise is spending money unnecessarily. I'd keep a few spares around in case they're needed, but don't feel you have to replace good springs just because they're used.
Generally, the first sign of weakening springs is failure to consistantly trip the bolt stop on an empty mag. Of course, this could also be a problem with the mag body (dented, dirty), follower (especially if it is not a new anti-tilt type, or has just recently been replaced), or bolt stop (dirty, difficult to move). But if other mags work and this one doesn't, and you've checked the other items, then replace the spring.