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.