There was a movie some years back in which a lawyer became unable to tell a lie. Always telling the truth caused him some real difficulties, blurting out the truth when something a little less then the full truth might have been better.
How do authors on SOL handle this issue when dealing with characters you wish the reader would react to favorably? Do you have parents teach children that that they must ALWAYS tell the truth? Should, for example, a truth-telling character tell their best friend that they really cannot carry a tune, that the dress really does make them look fat. If they fudge the truth in order not to hurt a friend's feelings, do they run the risk of letting that friend continue to behave in a way that others may find offensive? Should they warn the friend even if the friend cannot do anything to correct things?