Do i need to mud the top of horizontally mounted drywall if i am hiding the seam with quarterround?
If you don't mud the top and bottom, the bevelled edge will show up. At the bottom the baseboard would be at a slight angle and could screw up your mitered angles. The bevelled edge will definitely show up at the top with only quarter round mounted up there. Mounting the quarter round up there might be harder than you think.