I always checkin 24 hrs prior to flight departure...doesn't matter what airline! And with SW, just about every other person is going to be checking in at the 24 hr window, if they didn't purchase Early Bird Checkin!!! If you don't check in at that 24 hr window, you're going to get a boarding slot way back in the C group. If you don't mind being in a middle seat, in the back of the plane, fine.
There is absolutely no downside to checking in prior to arrival at the airport.