A well-known Talmudic passage is related in the tractate 'Ketubot' as to what one tells to a groom at his wedding in order to bring him joy. The rabbis who were the students of the sage Shamai state that one is obligated to tell the groom his opinion of the brides qualities- be it pretty or pretty ugly, smart or stupid, etc.
The rabbis who were the students of Hillel the Elder argue that one is obligated to tell the groom that his wife is Na'ah ve'hasuda, pleasant and kind (in other words, heap the praises upon her), regardless what your actual opinion of her may be.
The Shamaiian rabbis ask-- but if the bride is lame or blind wouldn't that tantamount to lying, which the bible forbids against?
The Hillelians retort that if you would see someone in the market place who just concluded a transaction, do you praise his deal even if you think it was a lousy deal or can you criticize it?
Here the Shamaiians were in agreement- indeed you do praise the dealmaker, and the Talmud concludes from here that a person should always strive to be pleasant with others, no matter what your personal feelings as to how you perceive the bride, a market transaction, etc, and this would of course hold even where you might consider such words to be untruthful.
Some commentators use this passage to define what it means to be truthful. Truth, they hold, is a relative proposition, not an absolute and objective fact that stands apart from the situation. The example given is where a soldier is looking to kidnap or to murder a person, and he asks a third-party as to the whereabouts of the would-be victim. In such a case they hold that it is not considered untruthful to deceive the soldier, since the concept of truth is only meaningful within a moral framework, which in such a case of murder or kidnapping is not a required ethic.