Sorry for the trouble that you got in DCA. You dont have to take your shoes off if they dont alarm anything. THis means that if you walk through and they send you down because in their infinite ability to judge shoes sole height and they have not alarmed the hand wand or the ETD machine, you dont take them off. If there is a hand wand alarm because of anything; Eyelets, nails, a bump with the wand etc. You will have to have them x-rayed. IF they alarm the ETD machine, they have to x-rayed. You dont have to stand in you bare feet or your socks though. IF you decline to take them off after they alarm, you may not be allowed to continue to your gate.