In the Dulles area I've found that when the medevacs are transitioning from peripheral areas of Dulles to Fairfax Hospital they will contact Potomac Approach on 125.8 mhz.
This frequency is used by planes transitioning from 4,000 to 2,000ft and the east downwind transition. The controller tells them which runway to land on. Then the pilots are asked to advise when the field is in sight. The controller will then tell them to contact the tower on the appropriate freq.
Another thing about the freq, there is almost always static on it so it makes scanning my comm air bank difficult.