A well designed spring will never deform plastically (that is, permanently) in the application for which it was designed. That being said, I would hope that my factory mags have well designed springs, but I still load mine at least one short. I don't carry concealed, however, or I would check the uncompressed length of the spring and load them to max. I'd then periodically check to see if the free length of the spring had changed. At home, I think 8 rounds in my 10 round .40 needs to see me through what ever happens!
Maybe I need to just do the free length test and see what happens ... it would be interesting. Does Wolff make springs for a USP40C?
Saands