The computer automatically assigns the seats on the sides first. 90% of people ask for aisle or window and get something assigned. I don't think 5 C/D will be assigned unless some "Seatguru fiend" asks for them.

Another possible downside is that you get served last as they work the 767 from the back at least on UA 922 IAD-LHR.

If your FA is middle-aged, English, hungover, and generally incompetent then you might not want to be served last.... She had managed to literally destroy the entire cheese plate by the time she reached row 5.