The sole purpose of a doc. check at DCA is to avoid being stuck with the pax at DFW. It's not the big deal it is if there is an international transit involved. US carriers tend not to sweat this too much if the pax can produce apparently valid docs. The microscope comes at the gateway.

Needless to say, it's worth doing your own. I suppose I'd rather be denied boarding at DFW than entry at HKG. But, I'd rather be cozy in a room in Honk Kong than have to sweat any document detail at all.