I always make town car reservations (online) as soon as I have the flights and resort booked. The company I always use (Tiffany) requires that you follow up the online booking with a phone call to provide a credit card # to finalize the reservation, but they do not charge you anything in advance, and you can pay the driver when you arrive in cash or credit (the card you booked with or another card). I make the phone call within a couple of weeks of booking the reservation, and then I call once more a few days before arrival just to confirm (they don't require that, but that's the kind of person I am!).
Anyway, I don't see any reason not to go ahead and book once your plans are solid.