Depends. Empty forums wouldn't take up as much space, for example.

You could do what I do and have one forum for all the hub entries: In my case, every hub entry I own has its forum pointing to http://developer.byond.com/forums/Crispy/Crispiness/. That way I don't use up quite as much server space; and besides, it's much easier to check one forum than to check a dozen. =)