Assumptions serve purposes in research, but if not careful they can cross the line from being an assumption to becoming a fact. If you are stuck on an ancestor, write down everything you know about him from memory (place of birth, place of marriage, place of death, occupation, educational level, social class, religion, etc. etc.) without any references to anything.
Then go back and see what you have some documentation for. Statements for which you have no documentation may very well be assumptions about the ancestor that are incorrect.