No aesthetic rule should be strictly followed, yet rules are there as a guideline for the achievement of that perfect look bonsai trees have.
The aesthetic canon of bonsai art has been gradually developed during centuries of trial and error by fine masters. Those rules are suggested by practical aspects of the bonsai cultivation.
The primary meaning of the bonsai tree is imitating it’s counterpart living in nature, achieving that well balanced look of a “normal” dimension tree. However, on a little bonsai, there’s not room for all of the branches of a big tree, so they have to be screened.