I met with the Dean of Admissions at the dental school I'm hoping to get accepted to next cycle about 2 weeks ago and she said they were looking for a trend in volunteering. She also said as long as the trend continues from high school throughout college then it was fine to include. Essentially, they don't want to see all your volunteering in the last 6 months so it looks like you're only doing it to get into dental school is what I gathered. This could be an oddity throughout the dental school world though so take it with a grain of salt.