When I was little there was one thing I was told not to do (besides play with matches, run with scissors, etc, etc...). I was told --Do not say "SHUT UP" because "BE QUIET" is just so much nicer.
As an adult I have stuck with this rule and I cringe when I hear the frowned upon phrase uttered.
P.S. I'm sure I wasn't allowed to do head stands on my bed, either!