It is often a matter of context and personal preference. I hold a Ph.D. and so it's not wrong to refer to me as Dr. Gregory. I only ever introduce myself that way when I am on a university campus in my professional capacity. I occasionally allow others to introduce or address me that way (allow meaning I don't correct them in front of third parties) in other contexts, but I don't do it myself. That's my preference for everyone with a Ph.D. and in fact for most medical doctors too. I don't see why, in a meeting of the neighborhood watch, we all need to be reminded about somebody's educational background. That said, if you constantly call the dentist on your block Dr. Whoever you won't be grammatically wrong, just possibly socially wrong.
If the person is a stranger and you're writing about them, go with Dr. Whoever. If it's a real person, ask them, in case they feel as I do about it. I can't resist telling you what my mother, who also holds a Ph.D., did once. Someone introduced her to someone else, saying "Mr. X, meet Ms. Y. " Mr. X responded "Oh, I don't believe in Ms., I will call you Mrs. Y" to which my mother sweetly smiled and said "Oh, if we're going to be technical you can call me Dr. Y" - as you might imagine Mr. X turned very red and excused himself from the conversation quite quickly.