Also, if you worry about the busses, you can always take a cab back from a park thats still open. We never get a car, and rely only on busses. But everyone once in a while - if we have dinner ressies or would have to take a cab, we get one. Its never been more than $17.
I don't recall seeing them at the MK, but I know for sure I have seen them at the other parks where the busses are.