i agree. there is a question on the AADSAS that asks if you applied to any other healthcare programs in the past. don't lie on this section.
but, there is not question that asks for all the tests you have ever taken. so if you took the MCAT but never applied, then there is no need to mention it. unless they ask you during the interview. the schools are looking to see you are sure about going into dentistry.
as long as you provide them a good explanation of why dentistry is the best fit for you, you will be ok.
(i took the mcat and applied to medschools some years back...it did not keep me out of dent school)