Whamming will not hurt your strings. You will eventually need new springs (in the back of your axe) however if you are worried about this try a Hipshot from www.Stewmac.com.

The only time your strings should stretch is when you first put them on.
After they are stretched in (after the first day or so depending on how hard you stretch 'em in), the next time they stretch will be the time they break !