Throughout this play, I believe Shakespeare favors honesty, and this contributes to the lesson he may intend to be teaching. Sometimes truth hurts, but it's necessary.
He creates Benvolio to be a truth-teller after every scene that involves a fight, and Benvolio always tells the story exactly how it happened. The significance of the truth is what it reveals about the two families. It seems that most skirmishes are caused by the Capulets.
In reference to the Friar, I think he was a sucker to see things improved through the families, was about to do it in a sneaky manner, and had a poor result. However, if you look at what Shakespeare does with his character in Act V, scene iii, the Friar is completely ready to accept blame for his part in the sequence of events that transpired. He tells the truth, and the letter Romeo wrote to his father verifies the truth. I think Shakespeare feels the characters have learned their lessons by the end and he allows amends to occur to create a resolution.