If you are opting for the checking in at 24 hr mark, I cannot emphasize how important it is that you get it in at EXACTLY 24 hours. We travelled from Buffalo with 2 DDs, 5 and 7, so we did not qualify for family boarding between A and B. I literally sat at my computer at 1:35 the day before, waiting for 1:40, the 24 hour mark. I pretyped everything in for the checkin so all I had to do was push enter when I got to 1:40. I am positive that when my computer clock said 1:40, I quickly pressed enter, and I STILL got B boarding passes (B9-11). I could not believe it!
I thought that we'd be OK, just go to the back of the plane and we could get 3 across, and DH could sit across the aisle. But by the time we boarded, it seemed like a lot of people had already filled the back! We somehow lucked out and found an empty row in row 10, with one empty seat behind our row in the exit aisle. Thankfully DH was able to sit by the exit door, and we had 3 good seats in front (although they do not recline, which is why I'm sure people left them alone). I was just glad I did not have to stress about this coming home as we flew another airline. I would hate to waste any precious Disney time trying to find a computer to check in! I would definitely pay the $10 for the advance option.
Still, it was a good flight, ahead of schedule, and everything worked out well. If I can find $59 flights again, I would fly SW in a heartbeat. However, for the same price, I would fly Jet Blue.