i worked with two dentists, shadowing one who has been my dentist since i was 6 or 7 years old and assisting the other who was my parents' general dentist. (he has since passed away.) i thought having their names made the statement more personal imo and maybe even a little easier to write. i don't think unless you ramble about how awesome Dr. Smith is for paragraphs the focus will shift from you to him. i think the PS should be about you, your desire to be a dentist, and the path that led you there. if a particular dentist contributed to that, by all means, acknowledge them, especially if it fits into the context of your whole statement. if you feel uncomfortable about it as a privacy issue, ask his/her permission before going ahead. i'm sure they won't mind though.