It will not work at IAD if the following two conditions are both true:

1. You arrive at IAD while mid-field immigration/customs is open.

2. Your checked bags are too big (or obviously too many) to go through the TSA x-ray machines for carry-on luggage. Or contain liquids > 100 ml/1L baggie.

You can't just "walk out the door" at IAD mid-field. The door leads to TSA screening as the only way to exit Customs. If all you've checked is an extra rollaboard, then yes, you're fine, just put it through the carry-on scanner. You clear TSA and then go back upstairs and you're still airside in Concourse C. Hop a moon buggy to the terminal and leave.

But if you've checked a full size suitcase, or sporting equipment, or a case of wine, the only way that bag is getting out of Customs is by the TSA checked luggage scanners that then continue the bags on their way to their final tagged destination.