How do you address an envelope to a Mr and Mrs Ben Johnson when wife is a Medical Doctor?

I think it really depends on the situation. No matter WHAT job she does she will ALWAYS be "Mrs. *First Name* Johnson" OR "Mrs. Ben Johnson."

I think the real question here is determined by the nature of the contact. For example:

1) If you were sending them a personal/business letter, a Christmas, Birthday, or other type of holiday card, AND you were sending it to their personal residence &/or mailing address, then honor her in her most important position/s as a wife/mother by addressing her as "Mrs."

2) If, on the other hand, you are sending this communication to her JOB, then address her accordingly - i.e. "Dr. Johnson" OR "Dr. *First Name* Johnson".