Well, of course you can use family search as a source as long as you're talking about the original document copies they have in large amount. I wouldn't use the family trees they invite you to construct except insofar as original sources in them are used to indicate or prove facts or connections. Of course even original documents must be only used in the context they provide. I.e. a death certificate might have errors as to parents or birth information.And a family Bible might have names misspelled, etc.