let it cool down for as long is it takes before you can reach around and undo bolts without yelling "ahh shit..ah..shit..hot" h:

but no you should let it get cool to the touch so that all the metals have contracted back to their natural resting. I dont know if technically it matters, but I always let things cool back down to their resting "form" before I start attacking...at least the head....as far as exhaust manifold and other fun stuff you have to disconnect before you can get into the valve cover/head bolts you can start that other shit as soon as you feel comfortable touching the motor.