SC,
The phraseology section of MATS Pt 1 says....
"Provided that no confusion or ambiguity will result the following words may be omitted from transmissions ;
'Millibars' when giving pressure settings of 1000mbs and above".
That said, with so many American crews using the airport/airspace I suspect most of us will say millibars the majority of the time irrespective of the actual pressure just to be on the safe side.
WF.