I have a theory about tipping. I have no evidence to support this what so ever!
Back in the old days when you had Kings and rulers and other royalty who had servants, a visiting dignitary would come and want to receive "special treatment" from the servants of the king he is visiting. Tipping or "gratuity" came out as a way of buying the servants services and silence, in a way in which the host king would not object.
For example, lets say Prince Charming is visiting King Trident, and Charming takes a fancy to one of Trident's chamber maids. He attempts to seduce her and she resists his advances. So Charming offers to her a bribe. The chamber maid refuses and threatens to tell Trident. So the next time they are all assembled at court, before the chamber maid can tell King Trident what happened, Prince Charming gets up and makes this grand speech in praise of the chamber maid and the excellent service she has rendered to him as a servant, and he would like to reward her with a token of his appreciation and pays her the money he attempted to bribe her with earlier. Now if she says anything to Trident she looks ungrateful, and as the only proof she had was the bribery money she is now holding, it would probably lead her to the guillotine. Since she took the money, Charming will later play the "your already in too deep, you might as well go through with it" card, and have his way with her.
Early tipping wasn't always buying sexual favors. More often than not it was buying loyalty. Since the servants are all in the service of King Trident, if Trident wanted to have Prince Charming murdered, he would just command his servants to do so. So tipping for good service with the promise "there's more where that came from, provided I'm still alive" was a preemptive move by Charming to get the servants on his side. Eventually this practice became so common place that all pretense was dropped and it became standard procedure to give servants a little something-something as a general incentive, seeing as you can't kill another kings servants. That's almost as bad as stealing their silverware or losing their horse.