I think of the same thing, you know, NNnneumann!
First of all I am not making fun of your name, I am just laughing along with sixto about how Seinfeld says it. I hope you don't take any offense at this.
With manifolds, cam and rockers in place I would expect the assembly to weigh well over 100 pounds. I have never found a machine shop that was very anxious to disassemble and assemble a head to the extent that J. Hidalgo describes. I personally would not want to let them do it even if they offered.
The typical auto machinist is just that, a machinist. Removing and replacing such pieces is not what they do best.
My preference for removing and replacing manifolds, cam and rockers while the head is tied to the engine is because the head will run all over the work bench while trying to do this. In addition removing these components turns the head into a feather to allow quick removal and more careful placement onto the deck of the engine.
Each to his own with their method here, but I wouldn't even dream of removing and replacing the head with all those components attached. I also wouldn't dream of leaving this work to a machinist and I have a really competent machinist.