My husband's N-400 interview letter said nothing about his pending I-751 or a combo interview. There was nothing saying I was required to be there. We knew there was a chance it would be a combo interview, so I went along. To be honest, I felt it was important for me to be there, to support him on what could be a huge event in his life. It did turn out to be a combo interview and he took the oath that afternoon! It would have been a mistake for me to not have attended. Since we were both there, it was easy for us to go out, when it was over, and celebrate!