It depends on how late you are going to be out, and what the park hours are for when you are going. Generally park transportation runs about 2 hours after the parks close, the boats usually stop a bit before the buses, if I recall correctly. If the MK is open, you can alway take the monorail to the MK and then the bus or boat back to the WL.
If you are out long after the park closes, I would recommend a cab, otherwise you'd need to take a bus from the Poly to a park that is open later or possibly DTD and then a bus from DTD to the WL