You can waste a ton of time trying to cut irregular ceiling lines on popcorn ceilings or you can take a few minutes and modify a 5-in-1 tool and clean those ceiling lines up. Place the 5-in-1 in a vise and use a metal file to put a notch in the end of the blade.
I tried various notches and found this one to work best. You have to take in to account the angle in which you hold the 5-in-1. I tried filing down to the bevel but doing that created binding moving the blade along the line so I filed to approximately the same distance away from the bevel.
Here you can see a very common ceiling line on popcorn ceilings. This particular ceiling has not been painted but the walls have. Click for larger view.
This room is 13’x14’ and I spent no more than 10 minutes running the 5-in-1 tool along the ceiling line. Now you can cut faster and have a nicer edge to work with. Oh, almost forgot… the homeowner will appreciate the sharp clean lines but the intent was to improve your time cutting. It’s simple and very effective.