I am not official person, but I asked same question, and here is answer.
http://www.termpro.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=35&t=000283
Rules are a bit unclear, but praxix is that even square subs are completely legitimate.
And since you can't get bigger area than square, other shapes are allowed (IMO).