I like to save a bank of points for last-minute trips and emergency travel, so I generally will pay cash for at least one leg of a trip that is less than $125. (Of course, I use my SWA Visa to buy, so I earn on the purchase.)
I often find that even on popular travel days, when no flight has a low price for cash, usually one of the non-stop flights will be less popular and still have a good price available using points. Any time I see one of those I tend to grab it.