It is also pretty important how much variation you get among all the cylinders. The less the better. It will also make a big difference whether you test the compression when the engine is cold or at operating temperature. When up to temperature, the readings are usually much higher (and more meaningful). You will be OK with the cast iron heads on the 3800, pulling the plugs hot, but it is not good practice with aluminum heads - the threads in the Al strip more easily hot. With aluminum heads, I usually use anti-seize on the plugs whenever I install them. Anti-seize won't hurt in cast iron heads, either.