Do you need permission from a business before posting a recommendation for their services?
Generally, you would not run into legal issues if you post a positive recommendation concerning a doctor's services. As a matter of good form, however, you may want to contact the doctor in advance...there may be professional reasons that the doctor would not want to be associated with your site or not welcome the publicity in that form. (Usually, though, people are happy for the good press).
Note though, that you could run into an issue if your site expands to include not just recommendations, but also evaluations (which may not be so positive) of doctors. You may want to consult with an attorney who specializes in defamation (libel) law so that you have proper guidelines for what you should and should not do.