My advice is to be careful. I did it once, and never again. When you put in your dates of departure and arrival, they can select flights for you any time that day.
What happened to me was, I put in that we wanted to arrive a certain day, and the flight it selected had us arriving close to midnight that night. Of course they don't give you the flight until you agree to everything, so by the time you see your flight times, it's too late to change. We were supposed to be flying in for a job interview on the date of arrival, so obviously arriving at midnight that night wasn't going to work. And trust me, they WILL NOT change it for you, not for an extra fee, not for anything. They do mention several times that you can't change the flight, and believe me, they are serious about that.
All that to say, if you're really really flexible, go for it.
aak