I think that to strangers, it's appropriate simply b/c I believe that each person should get to choose how they are addressed. Some people like Mr. or Mrs. Last Name, some Mr. or Ms. First Name and some just First Name.
I certainly would have appreciated the teaching as a child. All my friends KNEW to call my parents' their first names because the first time they were Mr. or Mrs. so and so, they corrected my friends.
My parents are very laid back about that stuff and thus so am I. That said, I NEVER EVER knew what to call my friends' parents. I was so very uncomfortable everywhere I went. I wasn't taught to say yes sir or no ma'am. Yes, please and no, thank you were taught, but to this day, I am mortified by something I did...
Story: I was in the car with my high school boyfriend of 3 years as his family. I called his mother her first name, and he came unglued!!! Absolutely UNGLUED! He wanted to know where I got off calling his mom by her first name. Well, he'd NEVER told me not to nor had she, and I'd always been to scared to ask her last name. I know that is strange (and for a smart girl, pretty stupid), but she was on her 3rd marriage, and I didn't know his step dad's last name. The only thing they'd ever been called was their 1st names.
I will never forget that. My son will say yes, ma'am/sir or yes, Mrs./Mr. so and so until he's told BY THAT PERSON to call him/her something different. I want him to be comfortable. To this day I still have no clue what my childhood friends' parents wanted me to call them. Luckily, one mother was eventually my teacher, so calling her by her first name wasn't an option anymore.