If I am going to DC, I am using DCA. I also don't usually fly UA if I am going to DCA because I am minutes from LGA. Can walk to Crystal City, VA to/from DCA, or hop on the Metro to/from DC.
Use IAD for connections if I am traveling on UA to places not served by EWR. Connect at DCA also when I am traveling on AA.
Also not a UA loyalist, if the above isn't obvious enough