I was jumpseating and was assigned a seat in the back. After checking in with the captain I also very politely introduced myself to the purser and gave her my seat number. After the door was closed, she moved me up to an empty business class seat. It is not an automatic thing, and I have been left in the back before when there were empty seats up front, but not normally. Either way it is a free ride, and I wouldn't dare complain about being left in the back.