Agree that Rt. 15 should be easier (traffic wise) than around the Beltway, especially on a Friday when rush hour can start early.
Here's another suggestion...... leave home early, say noon, drive down and visit the new Smithsonian Air and Space Museum - the Udvar Hazey Center. It's just a few miles south of Dulles and a really neat museum! No entry fee, $12 to park. Spend a couple of hrs. with the planes (there's a Concorde and Space Shuttle there among others), go up in the "control tower" which overlooks the Dulles runways. Then catch an early dinner nearby since Dulles food is pretty predictable (there are several good reasonable restaurants close - we can give you suggestions) and drive the 10 minutes or so to the airport, getting there maybe 6:30-7. This gives you plenty of time if something unexpected happens (traffic, car, weather, etc.). Totally different approach but if you have the time it would be nice start to your vacation!
Have fun however you go!