It's an old wives tale. Grinding the corn does nothing to smooth out the points and edges that are taken care of by a dentist. The whole reason for going to the dentist is because they have the tools and knowledge to give the horse a smooth chewing surface. Even if corn did grind the teeth down, it wouldn't end up level and smooth like they really need to be for the horse to chew comfortably. If the horse has an unusual way of chewing anyway (overbite, underbite, off-sides), the corn might actually make it worse and harder for the dentist to fix.