It really depends what you, or more importantly your users, want. Categories are usually used to group forums that fall into some broad overall subject together. Have a look at the phpbb.com board index page
. PHPBB 3.0.x is a category, 3.0.x Support Forum is a forum, [3.0.x] Convertors is a sub-forum.
Topics cannot be created directly in a category only in a forum. A category is just a container for forums.
Forums can be converted to or from sub-forums or moved to other forums or categories quite easily, so it would be possible to restructure the board if you don't get it right the first time.