It has been my experience that they are held for the FA. I unwittingly recently had an agent at DFW's Admiral's Club apparently override this and assign me the "D" seat in the last row, wanting to ensure the best shot at 3 seats to myself.
When I boarded and went to the seat, two flight attandents immediately and assertively started explaining to me why I did NOT want that seat, and that they would get me moved. I was very disappointed, but not about to argue w/ two FA's. I later overheard those FA's complaining...something to the effect "Can you believe that?!" and "Only in Dallas!!"

Now I attempt to get row 29 or 30. Keep in mind that the last 2 (or 3?) rows have the engine next to the window, obscuring the view and making it noisier, which I don't mind due to my Bose headsets.