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The key here is that learning that noise isn't caused by high ISO, it's caused by lack of light. Lifting dark areas where there is no light will always make a noisier image than properly exposing them, but of course as you know, this runs the risk of blowing out highlights. Again this comes back to knowing your camera and its capabilities.
But to circle back, no, there isn't any specific rule. The rules are set by you and what you want to achieve for your artistic vision in a given scene.
I wouldn't be afraid of post processing, especially if it doesn't inherently change elements in the scene. Up to and including compositing I don't feel bad about. Majorly removing huge elements feels weird to me and transitions more into digital art.
Remember, even back in the film days, the labs would make adjustments during processing to achieve a certain look, I see no reason why we can't and shouldn't be doing the same thing with RAW files.
