I think it depends on the situation. For example, if you know of someone who is cheating on their spouse, being the one to "drop the bomb" might be the worst possible thing to introduce to an already dysfunctional situation. Such intensely relational issues require serious, professional insight and should not, IMO, be entered into on a whim or out of a sense of righteous indignation, superiority, etc.
Re: cheating at cards, I guess I wonder if the gain ("exposing" evil) is worth the potential loss. Is potentially damaging a friendship, relationship, or otherwise worth it for something that, IMO, is pretty insignificant?
At the end of the day, I think we as Christians need to be extremely careful how we toss around our morality, especially in regards to passing judgment. Moral behavior is an extremely complicated thing, and cannot be reduced, in my opinion, to black and white calls that can be made with regular application with no reference to the circumstances involved. And at the end of the day, is any good resulting from exposing evil? Is it this that will draw people into God's kingdom, or is it the unconditional holding out of God's love? God may be capable of holding together mercy and judgment, but I am quite skeptical that humans are capable of the same…