If you're hanging drywall in a room where you plan on installing a dropped ceiling, you may have thought, "What's the point of drywalling all the way up the wall?" The answer is fire blocking. You have to have that to keep fires from spreading quickly through your house, and drywall qualifies. The good news, if you were hoping to cut costs by not drywalling that last foot or two, is that drywall isn't the only material you can use for fire blocking; it's one of many.
That means if you have fire blocking you can leave the top portions of the walls unfinished. . . unless your local building code says differently. As with any facet of a remodeling project, make sure your plans meets the safety standards of your local jurisdiction before proceeding.