If it is somebody that I judge not worth it to confront, then I don't. That goes for acquaintances and the like.
However, if it is someone important to me and plays a big part of my life (A or not) I definitely do confront them. I wonder how far confronting really took me in the case of the A though. Didn't really change anything ever, because he was still continuously lying up until the very end. Looking back on it I think the smarter thing to do would have just been to realize I was wasting my time even bothering to confront him or trying to have reasonable conversations about the lying with him. I think the more effective method would have just been to leave him.