Depending on 'how close' you are to Dulles is the issue too. Can't hurt to contact them and see what they say.
IAD Dulles Tower Phone 703-661-0895
" " " Fax. 571-323-6407
Just be sure to ask them if they are busy FIRST before you start a lengthy conversation. I've found most tower personnel are a friendly bunch, but they do have bad days like the rest of us. Common courtesy goes a long way. Explain what you want to do and where, making a point to tell them your maximum altitude during your flight.
As long as we all play nice together, there shouldn't be any issues sharing the sky.
I am also right on the edge of the 5 mile area but there are plenty of places to fly.
Depending on where you are located you can go to Algonkian Park or Baron Cameron Park. These are outside of the 5 mile radius.