We did this on our last trip. We actually intended to use it round trip but after we checked in and got in line for about 20 minutes, it seemed like we were going nowhere in a hurry and it was getting late, after a long day of trying to get out ahead of a blizzard so we bailed and took a cab. On the way out of line I asked the CM if the return trip would be okay and he said yes. Now unlike us, you know in advance so you can just go ahead and book it one way.