How does a conductor arrange the seating in a choir?

In my school, the concert choir is arranged with sopranos on one side, tenors and bes in the middle with tenors in front and bes in back, and altos on another side. Within the sections, it's by height. But in show choir, which competes and stuff, our arrangement has a lot to do with vocal ability and how well certain people blend together. If your group competes it might be slightly height-based but also skewed based on people's actual singing ability, if not it's probably by height.