Depends where you live. If you're in a blue area, leave it blank, because every census form that gets filled out increases the population, and therefore the electoral college and congressional representation.

My upstairs tenants moved out, and I got a census form for them. I ignored it, because they don't live there anymore. They actually sent someone to my house on a Saturday to inquire (my neighbors told me, I was in synagogue at the time). I do live in a deep blue area and I think they were trying to get the maximum number of responses for this reason; if I were in a red area, I get the feeling they might not have been so assiduous.