This. This this this.

Daraia hits it perfectly on the head. Be polite, be respectful, but ALSO be honest. On the flip side, be honest, but ALSO be polite and respectful.

Lying isn't tact. Tact is learning how to say the truth in ways that make it easiest (or at least, least painful) to accept.

"You guys suck, I'm out of here," is rude, even if honest. "I, uh, have to go water the dog," is dishonest, even if it is less rude.

"I'm sorry, folks, but this run is really beating us up. I'd like to call it, so if you want to find another tank/healer/dps go ahead." This points no fingers, but states what's happening. If someone asks, "was I bad?" you can answer honestly but politely. "You were doing okay, but your threat seemed a little low. Were you ?" Or "You were healing okay, but the tank seemed to be dropping a lot. It could have been his gear, or your gear... Were you OOM a lot?" And so on. Constructive, not rude.

On the analogy question, "Do these make me look fat?" is a Yes/No question. This makes it a trap if the honest answer is "yes", because one answer is mean, and the other is lying. That said, I don't blame people who say it for using it, as it's societally-ingrained for many people to be manipulative in this way... But learning to not be manipulative will benefit a person mentally and emotionally in the long run.

A much better way for a woman who would be tempted to ask "Do I look fat?" to seek validation and a positive boost is to ask a NON-YES/NO question: "How do I look?" Because the respondent can then honestly, truthfully and *supportively* answer "You look great!"