The baseline is to try to produce something that can be understood by your public, so try to stick to whatever convention they are used to, or come up with new ways that they can make a sense of.
But I won't even call that a "rule".
I don't think it needs to be exact, but an effort to express things accurately should be made. If I say, "Fish garbanzo bean flunky jellybean!" It is quite reasonable for you to say, "No, that isn't the name of an ocean."
If I continue doing so, then I would expect to be excluded from the conversation.
Really? Isn't that a bit broad? It may be half true for some languages but definitely not for all.
For example in Germany we have the "Der Duden" which is not only a dictionary that defines the correct spelling of words but also contains a huge set of grammatical rules. It's still the de-facto standard for correct language in German.
And even in english you'd probably agree that there are some rules, e.g. when it comes to the correct spelling of words.
Linguists on the other hand have given up on prescriptivism decades ago. Scientific linguistics are entirely descriptivist, studying how language is used rather than prescribing how it ought to be used.