I'll actually answer your question! Personally I always answer as a couple. I'm married, so doesn't make much sense to answer individually. If I was unmarried, I would answer individually (unless I was in a domestic partnership or something).
I also always wonder if I'm supposed to be answering as a couple or individually when I apply for credit cards/limit increases (no, I'm not in credit card debt, but I do believe in using credit cards to their fullest potential, hence the user name.) I always answer as a couple unless I'm told to do otherwise - I figure it's not lying, I want my income stated as high as possible, and if they clarify later then I can always correct the amount stated.