Yeah, not to worry. If you gave your arrival info to WDW when you made your reservations, they pass it on to the transportation company.
BUT when you get your packet with the vouchers, I always call the number for the limo company and confirm my arrival time, ESPECIALLY since the airlines sometimes cancel or change flights and so forth.
We usually fly in the day before our reservation starts and stay at an airport hotel to get rid of jet lag. Then have the limo pick us up at the airport hotel.
They don't mind, as long as its near the airport and not a long way from there.
every time I specified a pick-up time, they have been there early.
very nice drivers. last time DW needed some sinus meds and he stopped at a local drug store a few blocks away from the pick up location.