First, remember that McBean is a simulation, and is not perfect; that's why you have to measure the completed box and adjust the rear chamber accordingly. I've personally found that predictions almost always end up with a rear chamber too large.The impedance testing I refer to is electrical.
Usually backchambers prove too large and you end up filling them with bricks.
A deviation of 5 Hz or less isn't going to be critical, but 10 Hz high would give a response bump above the Fc with rolloff below it and 10 Hz low would give a more severe response loss at Fc.