A. Rob Johnstone: "I know this sounds funny, but don't miter the inside corners & cope them. Use a coping saw to cut the exact shape of the quarter round molding where the two piece meet. Then butt the cut molding directly onto on untrimmed piece of molding at the corner of your ceiling. When done correctly, the resulting joint looks like a perfectly mated miter. This does take a bit of practise, but once you get the hang of it, you'll be proud of the work you are doing. Outside corners are different, there you just need to "sneak up" on the joint, using test pieces to make sure that the pieces evenly divide the corner's angle (which is seldom exactly 90°)."