Anything in A is great and you will have no problems getting seats together and probably in a good location.
There are several factors that can lead to getting a higher number even at the 24 hour mark. First, A1-A15 are set aside for people who have purchased business-class fares or top-tier frequent fliers, so you'll never get A1-A15 unless you are one of them. Often, there aren't 15 people in those 15 spots, however, so A42 probably won't be the 42nd person to board the plane. Second, if there are people switching to your flight who have come in to your airport on earlier SWA flight, those folks were able to check in for your flight 24 hours before their original flight and would have gobbled up some of the lower A numbers. Finally, there are just a lot of people doing exactly the same thing you are at exactly the same time, so it's a bit of a luck factor involved.