I try for the seat at the rear where the fuselage curves in. The wall allows a better prop for your pillow and you get served meals first. You're close to the can, so you don't have to walk half way up the aisle to wait. The time will come where they charge passengers like they charge for parcel rates - the heavier, the greater the cost.
If it was a many hours flight, I would agree to share the burden by swapping from time to time.