Technically, by the book, yes it is supposed to by hot. Yes all the plugs are supposed to be out.

In my experience on hydraulic cammed engines with cast pistons hot vs cold doesn't really matter. And cast iron manifolds take FOREVER to cool off.

In my experience 1 plug removed yields the same results as all plugs removed.

Try it cold because its so much easier, and go one plug at a time if you like. If its low then worry about if you need to do it warm or not.

Now I need to pin those needles, got to feel that heat
Hear my motor screamin while I'm tearin up the street