I'm used to the IAD set-up where the terminals are before the immigration booths, so if you get the X, it's easy to cut in line up front and get processed, or just walk through with the X.

Last night coming in through JFK, I found it interesting, that they use a lane for the terminals and that the GE terminal was after the (staffed) immigration booth. I guess if you get an X, you go backwards to go in front of the guy sitting there, but it'd be a traffic jam if more than one person was getting an X or people were waiting for the machine (has yet to happen to me).