My .02...
Unless there are enough people on Dis to cover EVERY FA/gate agent AirTran has, I think the answer is "you need a sticker."
Situation 1: Everybody on Dis says "nope, never been asked". You get on board and aren't checked.
Situation 2: Everybody on Dis says "nope, never been asked". You get on board and get asked... no sticker, can't use the car seat.
Situation 3: Someone on Dis says "yup, they checked mine". You need the sticker.
Depends how you want to play the odds. Will you have a backup plan in case they refuse the carseat?