I think that for formal wear one should stick to the prescribed guidelines for kilt wear.
For casual I think anything goes, but one should know the rules before breaking the rules. It's one thing to break the rules because you don't agree with them or prefer a different look, it's something else to break the rules because you don't know the rules.
I think the kilt deserves respect and this respect is shown in learning the guidelines for kilt wear even if you don't follow them.