It's a good idea to spend at least a couple of weekends observing. Some schools specifically require a certain # of hours of shadowing but not all. All other things being equal (grades, DAT), some schools might use that as a reason to weed you out. As long as you have a reasonably intelligent answer as to why you want to do dentistry and have thought this through, you should be fine.
At one of my committee interviews I was asked the standard 'Why do you want to be a dentist' question. I told them I had worked in a dental office for 3 years. One of the professors said "and you STILL want to be a dentist?" Then they all laughed really hard for about 5 minutes.
That's good advice from Nycdoc. Let me add that having previous exposure to the dental field counts in your favor when you get an interview, because you can tell the interviewer up front that you are familiar with the field and wouoldn't mind being a dentist for the rest of your life!
When I was being interviewed for admission, I had no problems answering the same question everyone dreads: "why do you want to be a dentist?" I tell them I've been exposed to it all my life (dad is a dentist) and so I'm very familiar with the profession and would love to be a part of it. All the interviewers who asked ate it up LOL, especially when I started describing some of the procedures I've seen. The interviewers start thinking, "hey, this guy will fit right in. Let's take him!"
The candidates who usually get weeded out first are the non-committal ones who couldn't satisfactorily answer that question.