I hold my tongue until a certain point, usually.
I recently lost it when someone was being careless with their health (after weight loss surgery). I've seen it so often, and bitten my tongue, but I was just so over it, I told her exactly how stupid and dangerous she was being. Plenty of others were nicely advising her to make wiser choices, while many more were saying, "there there, you're trying your best!" (mollycoddling and encouraging dangerous behaviour IMO).
I bite my tongue at work though. Clients come to me saying all sorts of things, assuming I must somehow agree with them... sometimes it makes me laugh, sometimes it makes me want to argue till I'm blue in the face, but I just give them a non-commital sound and try not to encourage further discussion on the topic. lol.