You "could" do it that way, but the engine won't be spinning as fast, so may not be a fair test. If you do it that way and get a better reading, I'd trust it. If you get the same or a lower reading, I wouldn't trust it.
I agree it's not different by much. What does the manual say about permissable difference between cylinders? (I'd acknowledge here that afmcorp "craig" already mentioned this)

Gilly