Custom checks are usually done at the last port when you are going out of the country. If you are not going out of the secured areas within each airport, you may be asked to pass custom at your originating airport. But why are you concerned with that? US customs are not too bad, just don't say anything when not being asked, and keep your answers short and clear, like you are in a courtroom. You can tell that they are trying to make the security checks as painless as possible, given that it is already a pain.
What's really hassling is coming back from the states: they now scan everyone's shoes through the x-ray machine and do random personal checks. All electronic equipment, laptop, digital cams, palm, even cellphones are checked vigorously. So be sure to leave a lot of time prior to your flight departure.
Security on flights to DC is particularly tight. Once you are onboard the aircraft, you are literally tied down on your seat, you can't move, can't even go to the lavatories and they will not serve any food on the flight, at least during the last 40 minutes before landing in DC.