If economy, exit rows are definitely my favourite for these long haul flights. Be careful of bulkhead seats, some routes will at times have a massive amount of children which will be placed there. That goes for IAD at least, not sure about ORD.

Exit seats are given out free at first come first serve at check in, at least that is the official explanation.
Also note that ET loads passengers from front to back, so you will often find the first half of the plane very cramped while plenty of free seats in the back half.
If not an exit row, I'd therefore definitely have picked a seat in the far rear end, however making sure to leave a few rows between myself and the galley in the back. Some people like to think it's a bar area during these long haul flights.