As long as the engine turns easily with the plug in, you should be good to go.

To the original question: It is not HEAT that triggers the explosion, as David pointed out, it is a chemical reaction between the filament in the plug (Which is made of Platinum) and the alcohol in the fuel.

Heat only HELPS the reaction happen. So technically, an engine CAN start without a glowdriver (and I have heard of cases where this has happened).