Since SW is the only airline in Kansas City that offers non-stop flights daily, I have to book as soon as Southwest releases its dates. We also usually fly during busy times (spring break, Memorial week, summer), which makes a difference. I don't have the luxury of waiting, since experience has taught me that when flying during high seasons, it is best to book as soon as you find airfare that is palatable, IMHO.
I also like that SW never changes its schedule. I've know so many folks who book nonstop on legacy carriers that later change the flights to indirect or less convenient times.