I do them both ways, dead cold after sitting overnight in the shop and then warmed up to at least 180F. I do it at TDC usually but you can test with the pistons down in the cylinders like piston engine aircraft mechanics due
They remove all the plugs and use the prop to bring the piston up far enough so both valves are closed and due the test there Some will do it at TDC also but not all of them do that
I use a aircraft gauge now, two gauges and reset the first gauge pressure so it has 80 lbs. pressure with air going into the cylinder being tested at TDC and read the pressure lost with the second gauge going to the cylinders with 80 lbs. on the first gauge