The most basic rule of etiquette since biblical times — it’s summarized in the Book of Esther — is “As in the King’s house, so in the country”. What that means is, the “last word” on matters of etiquette is the etiquette followed by the head of state. Most countries have some sort of handbook or guide published by the appropriate government office — which might be the Lord Chamberlain, or the Secretary of State or the Chief of Protocol. State dinners, ambassadorial functions, senior governmental or business functions, and high society, will all follow that guide whatever it is. In the U.S. it’s “Protocol” (PROTOCOL: 25th Anniversary Edition
The Complete Handbook of Diplomatic, Official and Social Usage
Written by Mary Jane McCaffree, Pauline Innis and Richard M. Sand, Esquire — http://www.usaprotocol.com/purchase.html). If you want to be technically correct you would far better follow this than to try to resolve the conflicting advice of Peggy Post and Miss Manners — although to be honest, Judith Martin seems to have expended the paltry $24 to get herself a copy, and even seems to understand it.
What you need to note in following correct protocol for writing names on invitations, is that one’s formal social name is NOT the same as one’s proper legal name. A proper legal name uses ONLY the title (Mr or Mrs) and the surname (Mr and Mrs Smith). That makes it completely non-sexist, since you don’t use the first name of neither husband or wife. If you have two people with the same surname, then the senior one is Mr Smith and the junior one is — I kid you not — “Mr Thomas” (or whatever his first name is) This *is* sexist because the correct form for his wife is — again, no joking — “Mrs Thomas”. Nowadays even I would break with tradition and use “Mrs Jane” for the wife of a junior son.
The only time you would use both the first and last name together in a formal social situation is if you have more than one Mr Smith and more than one “Thomas”. The correct protocol gets increasingly complicated from there.
Correct form regarding women’s names has changed since the 1970’s, and I don’t know what the 25th edition says. It used to be that a woman who kept her own name was “Miss Smith” regardless of whether she was married or not, and if there was more than one “Miss Smith” present then the junior one was “Miss Mary”. I do know what the Canadian style guide says, and that says to a) use the title and form of the name that the individual prefers, but if you don’t know her preference then b) assume she uses “Ms” for her title and her own first name, and list each person’s title beside their name:
Mr Smith and Ms Smith (if you are not using first names)
Mr Smith and Ms Jones (if you are not using first names and she has kept her own surname)
Mr Thomas and Ms Mary (for a cadet branch of a family when you are otherwise not using first names)
Mr Thomas Smith and Ms Mary Smith (for circumstances that call for both first and last names both), or
Mr Thomas Smith and Ms Mary Jones (for circumstances that call for both first and last names botha and she has kept her own surname)
… and yes: Mrs John Smith, if that is how the lady refers to herself. My mother styled herself that way for her entire life, whether she was standing beside my dad or acting entirely on her own.