I don't think voicing one's opinion is a matter of "fact or opinion". It can be opinion backed by facts. It isn't always a defence mechanism. Usually if I express an opinion it based on fact, however peoole can disagree with the "fact" not opinion, bit make it look like its the opinion they are disputing. I think it's not only knowing yourself but self confidence and trusting yourself. I trust I don't offer opinions willy nilly and there is a reason for it. I believe in myself, facts and gut instincts. We don't just form decisions or opinions purely rationally and objectively, gut feeling can be a great way to discern fact from fiction.