My policy: if there is an incentive or cash reward then yes I will. Otherwise if someone comes up to me and asks to do a survey, or over the phone, I tell them that I do not answer surveys, personaly policy. It usually wastes more of my time than is necessary.
As far as the census bureau, I think the only things they NEED to know legally are how many people live in your household, and maybe what the household income is. Those long drawn out surveys they do nowadays that take like 3 hours are ridiculous, not to mention they come back every few months to follow up. No I won't answer those anymore.