Should drywall be hung horizontally or vertically?
Horizontal:  Pros will fit top piece to ceiling first and cutoff bottom piece for a 1/4 to 1/2 gap at floor.  The tapered joint is easy to fill.

You can also get a tapered butt joint by hanging the butt ends between studs and using a floating backer with a depressed center area that forms the "butt taper".  They are available commercially or can be homemade. This is the easiest way to get a "dead-flat" ceiling (or wall) when only the best will do.