This is the way SW boards it's passengers. Those that are allowed to preboard (handicapped, those flying with children under the age of 5) board first. When they are all on, then those with A boarding passes board, then the B's, then the C's. The A passes are nicer because you can get seats together that way. If you end up with C passes, your group may be scattered thoughout the plane.
As far as how early to call for those A passes...I flew in August, on a Friday from Boston to MCO...I called at about 4:30 a.m., just before we headed out for our 7:45 flight, and I was able to get two A's for my dd and myself.