Actually, always make sure the deck is cool, the engine is cool, especially since they are aluminum and aluminum will warp very easily if cold water comes into contact with running temperatures of the heads.
It won't happen every time, may never happen to you, but, it can and will happen to some.
During my early years of building with aluminum racing heads, we found that adjusting valves cold would change so dramatically when the engine came up to temperature we had more than one ocassion for valves to go into orbit and make contact with pistons. When the engines cooled down, the valves would once again have the clearances we set. Of course, once you have overcome the resistance of the valve spring energies, valves are going to go seeking another atmosphere. So, lots of things to consider in engine building as well as cleaning.
Let everything cool down before cleaning no matter which method you use.
I've cleaned the deck with the cleanout by starting up the mower and running it while shooting water and it cleaned it thoroughly. I also use spray on teflon after the deck dries. Works great.