On the subject though (yours) - in anything other than a Cessna 337, I would still CONSIDER the option of vacating (the aircraft) and let the bugger roll away from me.
On most aircraft there is a little knob called a Parking Brake. The insurance people would be most upset if your bugger rolled away and killed a passer by simply because you failed to set the parking brake.
Hugh FD. Must say I have never heard of "piston slap" as reason for using the mixture to simulate engine failure. The Lycoming Flyer Key Reprints published in 2002 and on page 50 has this to say:
In our publications we then explained the reason for using the mixture to abruptly terminate power. By putting the the mixture control in ICO in a normal open or operating position, the pilot merely cut off the fuel but allowed the air to continue to fill the cylinders with resulting normal compression forces that are sufficient to cushion the deceleration of the engine and prevent the detuning of the crankshaft counterweights. However, any practice of simulated engine out condition at low altitudes should be best accomplished by a slow retardation of the throttle in accordance with the NTSB recommendation. This careful technique will protect the engine and at the same time provide for instant power if is needed.
With the throttle wide open and the mixture is cut initial cushioning is effective as discussed by Lycoming above. Unfortunately the effect is nullified a few seconds later by the closing of the throttle as part of the identification phase. With closed throttle very little air gets into the cylinders.