i do that to them also before i ship them art. i advise against it because some guys don't know when to stop, and they want to know why and how they ended up w/ a string that looks like yarn. i'll still use leather on my strings, but only once every month or so, and i don't let them get hot - just warm. guys get hooked on it when they see how shiny their strings get - overdo it, and rip the fibers on the outside i think.