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I don't think people should lie not to be mean. First it's being dishonest with yourself and others and it means you don't respect them to hear your honest opinion. I think tactfulness is better than dishonesty.
Then what about comments that might be sexist, homophobic, or racist? People who say those are telling the truth, and are being mean. If they lie, it would benefit them more than if they were to be outright saying what they truly think, right? They could lose their job, etc by being mean like that. Saying mean things as your honest opinion doesn't mean much.