Not quite. If you are connecting to a UA domestic flight, you use the midfield (C concourse) customs whether or not you have checked luggage. If you are terminating at IAD and choose to ignore the directions to main terminal customs, then you must not have checked luggage, since it _will_ go to main terminal if it has an IAD destination tag. All posters are correct that if you terminate at IAD with no checked bags, you are much better off going the "wrong" way and using the midfield customs -- it is vastly quicker, especially if you are deplaning from F or C and thus will be at the head of the immigration queue. Deplaning from F or C and going to the main terminal, you lose your positional advantage, ... everyone waits for the same mobile lounge.