For a class, do what you need to in order to pass.
In the real world, I'd address a document to a female as Ms. Jones or Dr. Jones (if appropriate) unless I knew she used Mrs. or Miss. In my field, the assumption would be that she has a PhD, so I'd probably err one the side of adding Dr. unless I knew otherwise or knew that she preferred Ms. Jones, PhD. I would never address a document to Mrs. John Jones unless I were 100% positive that she wanted it addressed that way and even so, I'd probably default to Mrs. Jones.
Once a person has expressed a preference, I think it is most polite to adhere to it independent of what others think should be that person's preference.