Two good points from Dr. Janet Stemwedel: She’s discussing the use of honorifics in conversation (Dr, Mr, etc.) I believe personally it is best to start formal and then titrate it downwards.
First off, I think respect for persons ought to be absolutely central to our interactions with each other, every day and in every way. I’m with Kant on that one. People have the potential to do wonderful things — even of they haven’t delivered on that promise yet — and they have the potential to feel really bad if treated poorly by others. There’s never a good reason to go out of your way to treat someone poorly, and the extra effort it may take to treat the other people you encounter as actual human beings rather than just obstacles in your path is almost always worth it. (Sometimes the value comes in what kind of person it makes you to treat others well, but that’s still value-added.)
Let’s remember, … that respect for persons includes respect for oneself.