One caveat on Southwest, if you pay cash you will not get a refund if price drops, just a credit that you can use on a future flight (good for 1 year from the date of purchase). Also, they won't credit you automatically, you have to watch for price drops and switch your booking (like Disney).
If you book with points, then you do get points refunded if you switch to a lower price ticket.
I book SW as soon as possible with points, and usually they have a sale in early fall that I jump on. Last year price dropped 10,000 points for our 2 RT tix, which is enough for another 1 way ticket!