if working out of Dulles, the better bet is to list under Wash DC.  Technically, Dulles is located in Virginia being about 45 minutes from downtown Wash DC.  Most DC based clients will look to the Wash DC listings for providers working out of Dulles and other DC metro locations.  TER only added Virginia to its Ad and Regional boards fairly recently.  Prior to that, DC was the obvious choice when putting up an ad, but since Virginia was added, it has become a bit more confusing.   The Virginia listings tend to be better suited to providers working out of locations well outside the DC Metro area like Charlottesville, Fredericksburg, and Winchester.

But if rules allow you to do your ad on both , all the better.

I wish they would reclassify the DC Board as DC Metro

Dulles area while far west NoVa is still considered part of the greater DC area.
You can alternate weeks using the DC board and the VA board but if you can't have 2 ads on the DC and VA boards with the same dates running at the same time.  
Since I am DC based I don't have to use dates BUT I do have to put "visiting" dates on the VA board. PM me (or email me) if i've confused you lol.