I do my cut in a little different than most. (well, more than likely, all) I actually roll the wall color on to the ceiling a bit to fill the corner, and then, when I paint the ceiling, I'll do a tape line just shy of the ceiling on the wall and caulk the edge. I'll then paint the ceiling and when it's dry, I peel the tape off. This leaves a nice straight line between the wall and ceiling color and hides any waviness in the ceiling drywall. It takes a bit of extra time, but the results are worth it.