I'm afraid manners are not morality. Presumably a moralist may never tell a lie. Someone interested in manners tries to tell as few lies as possible because on the way back he might have to say the opposite. But it's very noticeable that the word "truth" is usually accompanied by words like "the awful truth," "the grim truth." Lies are never considered grim. It's never appropriate to tell the truth unless it's already very bland, very favorable. The main difficulty is, of course, the word "no." You have to avoid at all costs saying no. So when people say, "Will you marry me?" you say, "I don't feel I am worthy." It means "No."