Occasionally I use one of the online trees to give me a little boost or nudge when working on a family–especially when I need some general direction.
Sometimes they are helpful and sometimes not. Recently when working on a family in Stow, Massachusetts, in the 18th century, I took a look at the FamilySearch tree on the family. What I found had significant differences from what was in the will of the father of the family. At that point, I decided to focus on the wills and probate records of the families under study, transcribe those, interpret those and go from there.
Sometimes it is best to just start from scratch and work from the original records.
Learn more about research, methods, and sources in Casefile Clues.