An accuracy to 1cm would be disaster, although, when you are first starting out you wonder if anything more accurate is possible. The above poster is right about being accurate to within the 1/10 of a mm. And, the way to do it is with finger rests. Essentially, you hold the handpiece like a pencil, and you use your ring finger to rest on an adjacent tooth. Add to that lots of practice, and pretty soon you are removing tiny amounts of enamel, dentin, plastic, or stone. You'll get the hang of it pretty quick.