One of the many problems with this conversation is that, 15 pages in, no one has really defined what rules you're talking about. Rather than a "rules are guidelines" vs. "I don't need no stinkin' rules" discussion. What don't we redirect slightly, pick a commonly held "rule", and discuss when it is that we'd break with convention.
By way of an example, conventionally styled trees are normally tapered (or, as a "rule", trunks thicken toward the base and branches thicken toward the center). Most of us go to great lengths to ensure that we adhere to this convention. There are countless threads on this site that start with the question - "how do I get rid of inverse taper on this tree". I've even seen a video in which someone tries a very questionable technique of taking a chainsaw and cutting a groove up the base of the trunk so he could wedge the bottom open to correct inverse taper. Whenever one sees "inverse taper" it stands out as a flaw even (I assume) to the "art can't have rules so in your face" crowd. So - I'd argue that, as a rule, we there is a strong preference for conventionally tapered trees.
So, when would you break with this convention? Are there trees with inverse taper that still look natural? Can you show us an example? Would you always ignore this convention because you don't need no stinkin rules? If you did choose to break this convention, as an artist what image would you be trying to convey?
Scott