In our AskMartha mailbox, askMartha@marthastewartweddings.com, I fielded this question recently:
We are having a black-tie wedding, and we're setting the tone with our save-the-dates and invitations. I know tradition calls for formally addressing a married couple as Mr. and Mrs. John Smith, but I would like to reflect our more modern times and at least use the woman's first name. What is the best way to format this: "Mrs. Jane and Mr. John Smith" or "Mr. John and Mrs. Jane Smith"?
Unfortunately, the answer to this is "neither." When you are being formal, you use formal forms and formats.
The woman's first name is not included in a formal situation because she is being sheltered (not hidden) by her husband -- it is his role to present the family's official face to the world and act as a protective barrier. Under that idea, some might suggest you put "Mr. John" first (like the post office). Others say to never separate "John" from "Smith" (something about men not being torn from their last names), which means "Mrs. Jane" would precede it (that satisfies the "ladies first" idea we often see).
In fact, there are still people who believe that "Mrs. Jane Smith" is proper only for divorced women; to them, a wife or widow would always be "Mrs. John Smith."
Sure, it's archaic -- but that's formality for you. Nobody's going to arrest you, of course -- you can write anything you like. But if you want to be authentically formal, go with the age-old format.
What are you using on your invitations?