I don't think you have to book Supershuttle ahead of time for trips from the airport to your hotel. I've used Supershuttle from both LAX and Orange Co., and I've made reservations most of the times (perhaps all), but they are no obligation reservations (no pre-pay or even credit card). That way, you still have the flexibility to go with another service if a Supershuttle van has just left, and another service is loading with space available for your party. I'm not sure that having the reservations helped me get any priority over someone without reservations.
You do have to make reservations for the return trip back to the airport from your hotel.