Since the federal censuses are now available from 1850-1930 online, you should get them all. 1940 Census will be available in Feb 2012. Censuses are not perfect but you should check them. Often children will have their elderly parents living with them.
'Trees' online are only as good as the person who submitted them. Often they are 'sparse' trees meaning they only cover one persons lineage. Be very careful using 'trees.' Most have no primary source documentation and ones with it are often incomplete and wrong.
Primary source documents are ones recorded at the time of the event. They include vital records, birth, marriages and deaths (recorded recently at the state level); deeds (recorded in the county) Wills and Probate records (recorded in the county), censuses, newspaper obituaries, cemetery records, etc.
It is better to go back slowly generation by generation and get the primary source documents. Then you will be certain your research is correct.
The web does have some primary source documents on it. I did all my research prior to the advent of the web. It took me longer but I know it is correct. The web can be a real aid but realize without seeing a copy of the primary source document, you only have a 'clue' which needs further research.