It's a dog-eat-dog world with getting into dental school and like the majority have said, it won't hurt you. I sent them to 2 schools, because one gets 2,400 applications for 200 interviews for 80 seats and the other gets close to 5,000 for (who knows how many interviews) for 230 seats (obviously we know which one this is), so be courteous and take a few minutes to let the school know you enjoyed your time there, because I certainly did at both my schools.
The right way to word it or do it? Be sincere. At school A, I loved the presentation, thought my interviewer and dean were very informative (because they brought up info that I didn't find on the website) and was impressed with the clinics that I read were "sub-par" online. At school B, I loved that the dean disspelled some of the common myths, was interested in some of their 3rd year programs (that also weren't on the website) and thought the clinic was literally Steve Jobs' gift to dentistry (yea. THAT renovated).
I have no idea if this helped me, and it probably didn't, but on the slight off-chance that it did, I'm glad I spent a few minutes to thank the people that chose to interview me over the thousands of applicants that would send much more than small piece of stationary to get my spot. So do it. The worst that can happen is that it meant nothing, which puts you in the same spot you're in now, except you won't ever wonder "What if I did..."
Just my 2 cents and like any other opinionated response, it could be completely wrong! haha
***School A is also known for including hand-written notes in their acceptance packages, so I felt it be nice to "return the favor." I haven't heard back from them yet.
***School B accepted me, but it was a mass email that didn't have my name on it, so it was about as impersonal as they come and my letter probably never even made it to the building.