Not sure if the same holds true for D.C., but I was looking at fares on Southwest Airlines for December 18 to Orlando. When I first looked, one way was $162. I looked a few days later and the same flight was $229. We finally made up our minds that we wanted to fly and when I checked 3 days later, the flight I wanted to take was sold out. My other option was to book a flight with 1 1/2 hour layover in New Orleans putting me in Florida three hours later.
I guess it depends on what time of the month you're travelling and whether you have your heart set on a certain flight like we did.