This is a whole big debate I usually empty out the firebox. run th engine with the pump pumping to keep the water level up since the water consumption goes up with low steam pressure this also cools the boiler, wait until its down to about 5 or a little less psi the use the blow down vlv to help blow out crud from the water legs. normally we have steamed as long as possible on the day and need to get the thing cooled down reasonable fast takes like a half hour or more to get it cool enough not to melt the ratchet straps . I have a friend who shall remain nameless who drove his 1/2 scale case up on to the trailer under steam then chained it down, closed all the dampers and doors ect. and drove off down the highway turns out a 60 mph wind can really stoke the fires even though thought they we out. this set fire to the planks on the trailer bed he notices all this smoke rolling off the trailer pulls over and drains the bunkers on the fire. crisis averted