On the subject of etiquette, what is etiquette? I think that making someone feel comfortable is much more important than following a rule of calling every man "sir."
Here's a great example. At a formal dinner at Buckingham Palace, one of the guests had no idea how a finger bowl is used. (It's used to dip the fingers in to wash them after a meal) So, the guest picked up the bowl and began to drink from it! Rather than make the man look foolish, Queen Elizabeth did exactly the same thing. Technically, she was wrong for how she used the bowl, but she was the epitome of a lady with etiquette by putting her guest's comfort above etiquette rules.
In my opinion, addressing people how THEY want to be addressed is much more important than dogmatically calling everyone "sir" or "ma'am."
If a woman wants to be addressed as Miss, call her Miss.
If a woman wants to be addressed as Mrs, call her Mrs.
If a woman wants to be addressed as Ms, call her Ms.
If a woman wants to be addressed as Ma'am, call her Ma'am.
True etiquette is making the other person feel at ease, not calling her "ma'am" because you think that's how she should be addressed.
Beyond that, I would think it rather difficult to insult anyone with the appropriate "please" and "thank you."