For us, it depends on context. When responding to an instruction, our kids say "Yes, Mommy" or "Yes, Daddy." Or at church or to a teacher "Yes, Mr. Craig," "Yes, Miss Bonnie."
When responding to a question about preference, like "Do you want grape jelly?" they would say "Yes, please" or "No, thank you."
When responding to a question asking information, like "Did you like that movie?" then just "yes" or even "yeah" or "yep" is fine with me.
Also, in any situation, it's the attitude that matters more than the exact words. So if I say "Please go spend 10 minutes cleaning up your room now," I'd rather hear a sweet, cheerful "Okay" than a sullen, angry "Yes, Mommy."
When introducing our kids to new adults, we generally either ask what they would like to be called or say "Miss/Mr. FirstName." Or if they are very close friends, sometimes "Aunt/Uncle FirstName." We really are not comfortable with our kids calling adults just by their first names, BUT I am a firm believer that each person has the right to choose what they are called (this always comes up in stories about in-laws who want to be Mimi and G-daddy; unless they are choosing "Mommy" I think it's their choice). So if a friend really wanted our kids to call her by her first name, we would allow it.