our hands are gonna worry more about the cold than the head and deck will.
slap it on. I can't say that I've ever paid attention to temps other than when painting when doing wrench turning.
just rememebr after torquing, that you need to fill rad and cooling system, then run it till warmed then retorque.. and would be a good time to double check valve settings.
me? on a issue witha head gasket? it's fill with clean water, warm up and test it like that till I was sure I had no leaks into crankcase. water in oil is bad.. coolant in oil will wipe out bearings..
if it is freezing in your area.. remamber to drain block and rad of the water, or add coolant after you test it, and then let it circulate to make sure you don't have a pocket of cold unmixed water trapped in a lower rad or block.
( IE i always test a engine assy with pure water not coolant .. just in case). DO rememebr to add coolant / or drain it if freezing though...