it'll depend on dental schools.
for example, out of schools i have interviewed, I know that Louisville and western do make their decisions on the day of interview whereas Oklahoma has a meeting in mid-November when they decide for out-of-states.
Either way, thank you letter is a good idea since it's not going to hurt your chances but only help you. Let's say even if they do make their decisions on the day of your interview, there are few possible outcomes (this will also depend on school admission system). It can be 1) accepted, 2) rejected, 3) hold for further review, they will compare you with other interviewees in the future to see if you should get in or not. Although not every school will follow this system but in case if they do and if you fall into category 3, having your name reminded one more time and showing the professional outlook by sending a thank you letter will help you with future decision.
Also, if you are placed in a waitlist after your interview, then having your thank you letter can also help those people who have interviewed to remember you better and maybe they will try to help you out more (not sure how much it'll help but it won't hurt your chance for sure..)
and i wouldn't advise writing a thank you email since it's not very professional although i don't think it would really hurt your chance.
People will get in fine without writing them but it's a sign of your appreciation for those people who have spared their time to get to know you as future candidate. Write them not necessarily to increase your chance but because you really do appreciate their time
