Article I, Sec. 2, clause 3, U.S. Constitution, provides that the census shall be taken every 10 years for the purposes of apportionment of (1) direct Taxes and (2) Representatives to the House.
It also says the census shall be taken in such manner as Congress may direct. So, while the Constitution establishes the TWO purposes of the census; Congress decides the manner in which it shall be taken (e.g., by the U.S. marshals in the federal judicial districts, questionnaires mailed by the executive branch of the federal government).
In Federalist No. 54, last para, James Madison, Father of the Constitution, explains why it is a good idea to have a "common measure" [i.e., the number of people] for determining both the number of Representatives for each State and the amount of the direct Taxes each State is to pay: As the accuracy of the census depends on the cooperation of the States, the "common measure" discourages the States from overstating or understating the numbers of their population.
The Constitution is clear, and Madison confirms it: The purpose of the census is ONLY (1) to determine the number of Representatives each State is to get, and (2) to determine each State's share of the "direct Taxes".
So! The federal government has no constitutional authority to ask you anything but the number of persons living in your home. When they act outside their enumerated powers, they have abandoned the "rule of law" - i.e., The Constitution; and they have embraced the "rule of men". And they are the "men".
They have no constitutional authority to ask you how much you spend on heating, how much money you have, your disability status & how many toilets you have. See Jerry Day's (Matrix News) excellent video posted by our own C.W. Jovaras http://www.youtube.com/watch?v=RsDhkPym01k & Dr. Edward L. Hudgins' (Cato Institute) testimony at: http://www.cato.org/testimony/ct-eh072000.html
Alexander Hamilton understood that the People [that's us] are the "natural guardians of the Constitution", and he expected us to be "enlightened enough to distinguish between a legal exercise and an illegal usurpation of authority" (The Federalist No. 16, 10th para). In Federalist No. 33, 6th para, he said:
...If the federal government should overpass the just bounds of its authority and make a tyrannical use of its powers, the people, whose creature it is, must appeal to the standard they have formed, and take such measures to redress the injury done to the Constitution as the exigency may suggest and prudence justify...
Hamilton also said that acts of the federal government which are not pursuant to its constitutional powers are "merely acts of usurpation, and will deserve to be treated as such." (The Federalist No. 33, 7th para). This is what it means to stand up for "the Rule of Law"! Shall we defend our Constitution? Or will we cooperate with the lawless federal government in subverting it?