To tell the truth... or not?

I recall when I was dating PsychoMike. [If you are new to the site and haven't read it, you simply must] My two roommates knew that he was trying to hit on one of them and decided not to tell me because they were afraid my feelings would be hurt and I "seemed so happy." Like I was happier overhearing him leaving her a romantic message on the answering machine? To me, it was a FriendshipFAIL of epic proportions. Our relationships were never truly the same and we moved apart shortly after that. It got me to thinking about truth and the games we play with it.

So let's just see, shall we? Today on BnB, we're playing "To tell the truth". Do you or don't you?

Scenario #1: You're out and about, you see a good friend's significant other in what appears to be a non-platonic situation with someone else. You say nothing but then later your friend asks you if you had any clue something was going on... what do you say?

Scenario #2: A friend invites you to come listen to them perform. You go, they are terrible. I mean worse than Nicki Minaj rapping with the New York Philharmonic. [yes, that means bad]  They ask you to write a review for them to post to their blog... what do you say?

Scenario #3: Somebody you have been friends with for a long time but feel zero cocoa chemistry with just told you that they think you two should try and take it to the next level. You can't imagine where the sizzle is coming from and you don't want to lose the friend... what do you do?

Scenario #4: You are out of town on vacation and have a one-night weekend fling that nobody will ever (ever, ever) find out about. Problem is, you're in a committed relationship. Do you tell it on yourself or no? [For the record, refusal to answer this one tells me something in and of itself ;-0]

Answer one, none, or all but give me your thoughts. Have you decided to just be brutally honest, come what may? Do you make allowances for little white lies or sins of omission? Do you weigh the benefits of telling the truth versus preserving someone's feelings? Just curious, what do you do? The floor is yours...