The only incentive for getting on early is when the space in the overhead luggage bins is in short supply. OK, so that's 99.99% of US domestic flights, but if you're going to sit in that cramped metal tube for hours, why rush? Once I had a seat in the row right by the door (international flight, not cattle class), so I just waited until pretty much everyone else had boarded before I bothered to get up and go to the gate.