How to get a straight line when painting wall with fillet to ceiling
I've had mixed results with tape.  Paint will wick/bleed under loosely bonded tape, especially if your surface is rough and well-bonded tape can damage the surface especially if you leave it on too long.  A friend of mine likes Frog Tape, but I've never tried it. I just free-hand it and switch to decaf coffee for a day!  I got good pretty quickly.  and it's not like tiling or something where you can't fix your mistakes.   If you don't mind doing touch-up the day after, it's usually faster than taping.

As far as positioning a line that goes with the flow of the ceiling, I would make a jig by screwing a few blocks of wood together that holds a pencil.  Use it to mark the wall and you'll have a visual guide to follow when you freehand. The yellow arrow in the sketch below represents the pencil and I would use a big enough block that averages out maybe a square foot or so of ceiling.  if you see sections of the line you don't like (because the jig hit a bump or whatever), just erase the offending section and use a long straight-edge like a 4-ft level or strip of trim to redraw the line.

Edit:
If your ceiling is very uneven, attach three (not four) "feet" to the top of your jig.  Doesn't really matter where, just position them away from each other in a triangle. This will prevent the jig from rocking on the high spots.