Cylinder head temperatures do climb some after shutdown. Most of the time after the engine has been working to its limit, a minute or two of idling brings the temp down enough so that on shutdown you don't start boiling. On my truck, if its above 105F out and I've been driving at highway speeds and pull in someplace and park, if I don't idle a few minutes before shutting down, I see a spike from about 190F to about 220F fairly quickly right after shutdown and can start my unpressurized system to slightly boiling. (I do run antifreeze) A minute or two of idling and this doesn't happen, but there is still a rise, just not enough to boil anything. I try to avoid shutdowns without idling unless its cold outside and the thermostat is already closing as I'm parking. Even in the wintertime the temp rise on shutdown is still there, just doesn't get to the boiling point. I suppose if the temperature gets high enough quickly enough it can stress something enough to potentially crack something.
