I did mine in a real mixed bag - depending on whom it was going to...
People who were married were generally
Mr & Mrs Robert Doe
then there are some guests whom I knew this would upset b/c of the women's liberation movement so they read
Mr & Mrs Robert and Jane Doe
Then there are those that were for families,
for the most part they were addressed to
The Doe Family
but... in the case of some blended families (where the kids don't all have the same last name)
Mr and Mrs Robert Doe
&
Leah, Alex, and Seth
awkward - but you need a solution
If people were living together I put
Robert Doe and Jane Smith
but if they were dating and not living together they received separate invites addressed to their respective locations and we instituted the + guest rule. So it would be
Jane Smith and Guest
We did them on a case by case basis. You know your guests and will generally know how this would be accepted.
There are so many different living situations, that a blanket rule is far too difficult.