I would add to that that even if a school requires you to submit parental information but you know that you won't be getting any money for whatever reason, I think most schools are pretty good about giving you leeway with their own grants, as long as you can document extenuating circumstances through testimony of a third party (attorney, social worker, clergy, etc.). From what i understand schools have gotten burned in the past by students whose parents claim that they won't contribute to their child's education but turn around and pay for everything anyway after the school has given the student a grant.
Foxy is right, though, it can't hurt to put down the information, especially since most schools require that parents submit their financial information anyway.