Unless you're volunteering at a dental clinic or helping with dental needs it really doesn't matter. I helped at the admissions office of a big dental group and I can say for a fact that schools nowadays are either looking for more than a padded resume and no amount of "BS volunteering" will guarantee you getting in. What worked was getting your admissions packet in before everybody else and having all the
prerequisites that the school asked for. If you're going to show something extra, make sure it really counts as schools don't give a damn about volunteering, specially when applying to advanced standing because you're already a dentist- why would volunteering as an assistant or "shadowing" impress them? If you can show CE certificates from implant courses or rotary endo for example, these would set you apart from the rest of the applicants.