I have a quote for plaster for an attic room. He said that he would round the transition from the sloped ceiling to the flat ceiling because it would be really hard to keep a straight line with a sharp angle.
Rounding will allow room for error in the framing. If you want to check just run a string along the angle. If it is off by more than 3/8 or so between studs stick with the rounded corner. I think it adds interest to the room but if you want that straight line look you could paint with two different shades using a tape line to divide the top and bottom.