I booked a limo in advance one time, just one way. Never heard of having to book round trip.
You could do a walk up, one way, and they tell you the price right then. I remember Larry saying something to the effect that when you book in advance, the driver is there waiting and the 'trip' clock starts at the time your plane lands, which wouldn't be a problem unless you want to make use of the hour you paid for by doing some sight seeing before being dropped off at your hotel. If it took a long time for your luggage to drop that might cut into your time.
But Larry (lhfried) does have the best take on limo matters. He might even be able to 'book' your ride, and he is a very nice guy.