I had a SWA pilot look it up one day, and from what he read, you are allowed to pre-board when your boarding pass is for the actual flight deck jumpseat. Otherwise they can tell you to find your place in line with your boarding group. Most of the time I get away with pre-boarding when I have a seat in the back, but it always depends on what mood the agent is in, of course. I use SWA all the time and they always treat me well!