Visiting a school is just that, you're visiting a school. Most of the time it wont effect you negatively, but it will unlikely effect you positively. I really dont know what dental schools tours are like but if they dont show you the clinics and labs then you're wasting your time. It's what's inside that really matters.
Normally if you're not in an open house, it is unlikely you'll be able to speak with a person on the admissions committee, ergo you won't be able to show your interest. But you can at least take a look at the area and see if you like it and possibly get some talking topics for whenever you get invited for interview.
I personally wouldn't and didnt do campus tours because I feel like it is a waste of time and resources IF you weren't planning on doing something else in the area. (To be clear, anything that puts you in touch with admissions staff are great, including open houses. Just not campus tours.)