Herbert,
You should change a fret when it buzzes against the next fret 'downstring'. It might be possible to go a little longer by pressing harder on the string but this breeds bad habits.
A new fret gives a minimal surface meaning the least dampening of the string. This translates to more overtones, a little longer ring time and overall note clarity, especially when using gut. Jacob Herringmann told me he usually refrets before concerts.
Sean On Jan 9, 2011, at 1:04 PM, Herbert Ward wrote: Do you need to change a fret if it looks OK under a magnifying glass? To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html
