To summarize, there seems to be some consensus that a warm engine can be started without applying the glow plug heater. Therefore, it pays to be careful flipping or turning the prop on a warm engine. OK, that makes sense to me.

Now, how many other than combatpigg think an engine can start by just putting on the glow plug heater (i.e. without flipping or turning the prop)? I suppose the theory is that there is some unburnt fuel in the combustion chamber that ignites when you put the glow plug heater on, starting the engine. This strikes me as so unlikely as not to be worth worrying about. If it could happen to a warm engine, why not a cold engine that's been primed? But what do you think?