I find that DL tends to hold some seats back. I regularly fly DL1/DL2 JFK-LHR-JFK (752 from EWR? No thanks, I'll just take the DEQM hit...), and always end up calling them for seat assignments after I book, as the DL.com seat map is fairly balky in my experience. I very rarely do not snag an exit or bulkhead seat.