I don't know for sure but believe I read somewhere that it's based on when you booked. The earlier you reserve the better the seats. But there's probably an element of luck or Cast Member choice too.
When we went two years ago (and booked maybe a month out) they started to take us upstairs. My mother, age 70 said no way to the stairs and they put us in the middle of the bottom section. No problems and they were very nice about it.
Sharon