I agree if an employee needs to fight for leverage over their manager then hiding your notes is as good way to do that as any. But if you need to use this tactic it's a clear sign the workplace is not healthy.

So, instead of as general advice "never show your notes" - I would rise to a level above - first in good faith, but in a defensive posture observe if your employer rewards co-operation or selfishness - and then choose your tactic accordingly.

Falling under incompetent management is a double injury - the incompetence is both professionally reprehensive and being managed incompetently just hurts.