AirTran allows you to get on their web site and pick your seats 24 hours up to 90 minutes before your flight. We have done this numerous times with no problems. I have read recent posts about people getting on to pick out seats and have found that seats have already been assigned to them. From my understanding you can override the seats assigned and pick out different ones if you want. It's a really easy set-up to pick the seats and print your boarding passes.
I agree with everyone that AirTran is a good airline with newer planes and really great employees. However, I must add that the one complaint I have (and many other posters) is that they are always changing their schedules.
I suggest you take the first flight out in the morning since they don't usually move this more than a few minutes either way. Coming back is another story. Last year I debated whether to take the 2pm or 4pm flight home. I figured I wanted our last day at WDW to last as long as possible so I picked the 4pm. Turned out it was a good idea since they cancelled the 2pm. Sometimes they cancel flights, but normally they change their departure time.....sometimes by quite a bit. Keep an eye on their on-line schedules even after you book. The trip before last year they changed our flight time and I found out via their web site. They eventually sent me an e-mail about it but it came way after I found out about the change.