Commpression test ?
Usually you would do them one at at ime so you don't get any wires mixed up but you MUST also disconnect the high voltage lead at the coil!
If you pull all the plugs at the same time you would be safe disconnecting the high voltage lead at the coil also. The engine should turn a couple of revolutions for accuracy, note the reading, release the pressure, and move on to the next cylinder. A warm engine would be most accurate.