At IAD, due to the general madness there and unpredictable delays, I generally give in and have my shoes off by the time I get to the belt. I just don't have time for a secondary grope-down and need to get through security as quickly as possible.

Times that I've been on a less-tight schedule, I have tried to go through with shoes on of various sorts--from sandals to tennis shoes to my comfy Rockport shoes I wear to work. I've been told to remove the sandals, allowed through sometimes with the Rockports and tennis shoes, and once been allowed through with tennis shoes and then been sent back through by a different screener than the one at the WTMD to put my shoes on the belt (after the one at the WTMD let me through!) I've never actually gotten the grope-down at IAD (only at MSP, MCO, JAX, and XNA over the past few years), although I'm sure if when told I had to remove my shoes I flatly refused, I would have been sent over.

Of course, IAD's video screens have people continually advising you remove your shoes, and then some TSA guy will always shout over it, "You must remove your shoes, shoes off, please!" Great consistency!