Hi,
I believe it depends on the school you are aiming to. Most clinical schools will want to see that you have good exposure to the field. Research is optional. But with research school, research experience is a lot more imperative. (of course there is always exception). For both kinds of school you still have to prove that you know what you're getting yourself into. I do not think you have to do dental research to qualify. As for me, I did cancer and cardiovascular research, and that was enough to get me into UPENN. I think what they're looking for is the ability to carry out research projects and pursue your interest, not specifically dental-related.
Good luck and take care. 🙂