As for warning signs we had a sign in our workshop at work, that stated.
(WARNING THIS SIGN HAS SHAPE EDGES)
Getting back to the origanal post. The piston in a single cylinder engine should always stop when the compression in the cylinder over comes the inertia of the crank/piston, therefore the exhaust port/valves should always be closed when you stop an engine. I never block the exhaust when I'm washing my bikes I just start and run them to make sure that, all is sweet.