If you're looking for answers to specific questions -- birth date, naturalization status, place of birth, number of children -- then certain census years are better to check than others:
Research Tips for the U.S. Census
1790, 1800, 1810, 1820 and 1830 Census - Listing only the head of household, along with a count of family members grouped by age and sex, these early census records are most useful for identifying your ancestors in a particular locality, so you know where to search for further records. These censuses can also be helpful in identifying immediate neighbors who might be related, locating possible relatives with the same name and spotting surname spelling variations.
1840 Census - While similar in scope to previous census years, the 1840 census is special in that it identifies the names and ages of Revolutionary War pensioners. A further search of revolutionary war sources for these individuals may uncover a wealth of genealogical clues.
1850 Census and 1860 Census - Often referred to as the first modern census, the 1850 U.S. census was the first to identify all family members by name, as well as identifying their birth place. An indication of real estate ownership suggests that land and tax records should be searched. Questions for the 1860 census remained essentially the same as 1850, so start with whichever of the two is closest in time period to other information you have on your ancestors.
1870 Census - The 1870 census is the first American census in which parents of foreign birth are indicated. This is very useful for identifying immigrant ancestors. Naturalized immigrants are also identified, suggesting followup in court and naturalization sources.
1880 Census - First and foremost, a complete transcription of this census is available for free online searching at FamilySearch.org, making it a good starting point for individuals without easy access to other census records. This is also the first census to state relationship of individuals to the head of household (keep in mind that the wife may not necessarily be the mother of the children), meaning less guesswork when determining family relationships.
1900 Census - My favorite census year, the 1900 census is the only available census which identifies the exact month and year of birth of each individual. The 1900 census is also the only census to record the number of years a couple was married (making it easier to identify someone as a second or third spouse), and the number of children born to the mother as well as the number of those children still living (making it easier to match children up with the correct mother in the case of multiple spouses). Also the first census to identify the year of immigration of foreign-born individuals.
1910 Census - Not one of my favorite census years since the quality of the microfilming is so poor in many areas, the 1910 census is still especially useful for identifying and/or verifying Civil War service as it identifies veterans of the Union and Confederate army or navy.
1920 Census - The 1920 U.S. census is especially useful for tracing immigrant ancestors because it identifies the year of arrival and citizenship status (alien, first papers, or naturalized) of every foreign-born individual, as well as the year of naturalization for those who chose to become U.S. citizens.
1930 Census - Unless you have access to the every-name index on Ancestry.com, the 1930 census can be difficult to search without a specific place of origin. This is because the traditional soundex index only covers a handful of states (all in the southeast U.S.). The best census to search when trying to connect living relatives to deceased ancestors as many of you will be able to find your grandparents living as children and young adults in the 1930 census.
