Look at what the policy covers, how often you can use each service, and how much do those incidents cost. Obviously, you won't have need of everything in a given year, but you might notice that the premiums exceed any reasonable benefit. You might also observe that the most expensive procedures are well known in advance. This might give a clue as to exactly when you might start such coverage.
Read the policy, find out if your dentist accepts that insurance, and pay special attention to any commentary regarding preexisting conditions.