From 540 CMR 4.00 (http://www.mass.gov/rmv/inspect/540cmr400.pdf):
(2) Inspection of the Vehicle. All inspections must be performed in accordance with the applicable
provisions of 540 CMR 4.00 and 310 CMR 60.02 by licensed inspectors in the inspection bay or
area approved for inspections by the Registrar.
(a) Inspectors must first collect the proper fee for the inspection.
(b) License plate(s), shall be undamaged, securely mounted, clean and clearly visible. No bumper, trailer hitch or other accessory may interfere with a clear view of the license plates. The license plate must be mounted in the proper location on the rear of the vehicle if the vehicle has been issued one plate. Both license plates must be mounted in the proper location on the rear and front of the vehicle, if the vehicle has been issued two plates. Any decorative license plate or license plate replica not issued by the Registry of Motor Vehicles on which any jurisdiction name appears must be removed from the vehicle.
I couldn't find a definition of "proper location" though. I may have to inquire and bring along a copy of the CMR.