The purpose of the census is to determine the number of representatives of each State so there is no "taxation without representation."  It was to be 30,000 people per Representative, but it is now merely proportional as the number of representatives has remained static.  Before the 16th amendment and Revenue (income tax) Act of 1913 were enacted under dubious circumstances, the federal government could only tax the States in proportion to their population when some "emergency" required federal expenditures in excess of the duties it collected.

The vast amount of private information collected during a census has become a means of justifying all manner of federal, state and county regulations, bureaucracies to enforce them and taxes to maintain the bureaucracies.  One way to fight government growth, and save yourself a lot of time is to refuse to provide the information.

http://www.lewrockwell.com/orig8/barnett6.html

[Article I] Section.2 [Cl.3] Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of years,  and excluding Indians not taxed, three fifths of all other Persons.  The actual Enumeration shall be made within three Years after the first Meeting of the Congress of the United States, and within every subsequent Term of ten Years, in such Manner as they shall by Law direct.

[Article I ]Section. 9.[Cl. 4] No Capitation, or other direct, Tax shall be laid, unless in Proportion to the Census of Enumeration [count] herein before directed to be taken.
 
Thus a lawful and legal response would be:

"I am to be enumerated as a free Person residing at this address. I am not an excluded Indian, nor 3/5 of another Person."
 
Then “Take the Fifth” :
 
ARTICLE. V
 
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual ser vice in time of War or public danger; nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.