You don't tip pitbosses. As a result, pit bosses actually make less money at many casinos than the folks they boss. There's a big fight going on at the Wynn casino in Vegas, as senior management has decided to "correct" this problem by having a portion of tips taken from the dealers and given to the floormen and pitbosses. As you might imagine, the dealers aren't too happy about this.
Anyway, when it comes to table games in a casino you should only tip if the dealer has helped make the game fun, been entertaining or helped explain the game to you. If they have been a "robot dealer" no tip, no matter how much you win.
A typical tip at roulette is to have the dealer "keep the change". So if you are a red chip player and cash out a winner, toss the dealer any silver you get. A green chip player would toss the dealer anything less than greens, etc. Of course, if they dealer has been especially helpful/fun, and you cash out a big winner - larger tips are always appreciated.