You do not HAVE to have a reservation. I don't think Disney would turn down any money you wanted to give them. But if you don't make a reservation, they may not have any availability and you will be paying rack rate. If you make the reservation ahead of time, you will most likely find some kind of code that will get you a discount.
For room only reservations, only one night's rate is required at the time the reservation is made. The balance is due at check-in.