All of the last-minute options depend on how full your flight is. While I do not doubt that some people have gotten good seats at the last minute, the flight we take PHL-MBJ has been totally full the last couple of years so people with no prior seat assignments have had to take what they could get - meaning scattered middle seats in the back.

I book well in advance and am willing to pay the few bucks extra to get a window seat up front.