It's very important to check maps. Boundaries change over time. Be sure the area where you think your ancestors resided is actually the area where they were.
A person may not have been living on the day the census was actually taken (not the official day). However, all information is to be "as of the official census day."
The U.S. census is taken every 10 years on a designated census day by an enumerator in a specific area called an enumeration district (E.D.). The first U.S. census was done in 1790; there are no U.S.censuses before 1790. The 1890 census was destroyed. Census information is kept confidential for 72 years after the census is taken.