No tree site should ever be used as a source. The source is where you find the documentation that proves the lineage, such as census, birth, death, wills, marriage records.
If you use a tree site as a source, and someone makes changes, how are you going to verify those changes?
If from a tree site, someone has an erroneous tree, and you copy it... then you are also passing on those errors. Do you want to know how many times I have heard "Its on XXX site so it has to be true." When I'm looking at the documentation that says otherwise.
Do you own research, and source it with proper documentation yourself, add it to your database. Then you know if your research is correct or not. I like family search.org because their sources are more user friendly for databases.