A. The hardware on many guitars is made from soft cast metal. This means that the tough steel of guitar strings can cause wear in the metal. Unless the grooves are deep you shouldn't see any loss in tuning stability. Just keep an eye on the areas on the top nut where the strings make contact. Also, be gentle when you tighten the bolts on the top nut. Forcing them will only make the problem worse.
Q. The locking top nut on my guitar has developed grooves in the metal that appear to have been cut by the strings. Should I replace it?