Most "bad" art advice works if you understand why it's bad
Shading without black is the one everybody repeats, and I actually do it most of the time. Not because black is wrong. Because avoiding it forces a small decision at every shadow, and those decisions add up to something more considered.
That's the thing with so called bad advice. The rule itself is usually nonsense, but the constraint it creates can be genuinely useful. The advice works for reasons nobody bothers to explain when they pass it along.
I suspect most of us quietly follow at least one piece of advice we'd never defend out loud. Curious which ones stuck for you, and whether the reason you keep them has anything to do with the original justification.
2
I agree. Stay true to your artistic style, remain open to feedback, and learn from the “rules”—but ultimately, you make the final decision based on the artist you want to be.