I just used them in June for a tranfer from WDW to Universal. For their longer trips, Sanford - WDW or Canavarel - WDW, they include a 20% tip in the price. For the local runs, incluing to/from MCO they do not.
My policy would be, if you just go from point A to Point B, eith no extra stops or special things, then if tip is included, that's it. If the driver makes a side trip for you or does something extraordinary like sticking around for an extremely late flight, then I'd though a few extra $$ his way.
Enjoy your trip !!