It can depend on equipment too. Our 737s were not allowed to be operated in QFE after they got retrofittet with EGPWS, even now in factory fittet EGPWS planes it is not allowed as EGPWS in that installation cannot work with QFE. So we have to calc back and forth. To enhance crew awareness it is mandatory to document every altitude clearance in writing, even below FL100 if we are to operate in QFE regions.