When do you just let people be wrong?

I spent this past weekend with some family I don’t see often, and as is often the case with older family members, I got a lot of well-intentioned advice. There were many opinions stated that I disagreed with (e.g., hide anything you value because the maid will steal them), and some factual statements I believed to be untrue (e.g., unscientific ways to boost my immune system).

I have a tendency to be an obnoxious know-it-all so I tried to just keep my mouth shut through most of these. But it got me wondering: how often do you find yourself in the position of hearing something you completely disagree with, or know to be false, and you just let it go? When do you try to politely disagree or correct the person, and when do you smile, nod, and change the subject?