Since I have been flying nothing but SWA the last 4 years, I usually book as soon as they release their schedules for the time of my trip, which is usually about 4 months ahead.
SWA often has sales, and if the fare for my flights drops, I re-book at the lower fare and get a credit for the difference, which I can apply to next year's flights.
I say, wait. If SWA flies out of your home airport, your rate will almost certainly drop after SWA releases their fares for early Dec, saving you money even if you don't book with SWA.