For most fares, IAD and DCA are considered coterminals for flying in/out of. I've done it in the past and recently called regarding this policy and it was confirmed that I could do it on my ticket.

Best bet is to call to see if your ticket restricts this. If you get some funny answer, call back later and try again.

When/if you do go to DCA, check the loads before you go with an agent to see if you have a good chance of getting on standby.