If it is a lawnmower or minibike motor that has been run at full throttle for quite some time and is quite hot, before shut-off I run the throttle gradually down to turtle speed (says so right on the throttle knob) and let it idle for a minute to 30 seconds* maybe, then kill it.
It's 'cooling it off' and is just a superstitious thing I do.
*I use an inline fuel shut off valve, if the engine has one, and let it run dry.