If you have the headroom, then yes install the subfloor. Plan on at least 7-1/2 feet of headroom when its all done - including floor and ceiling. A drop ceiling should have 6" clearacne to the joists to make light fixture install easy. Don't have the headroom - then skip the subfloor.
Is a subfloor needed? I didn't install one. I have an 8-pound pad and tight-loop berber carpet. My floor (zone 5) is warm/cool (NOT COLD) in winter. Very comfortable. The floor is softer than the upstairs on a subfloor.