Where the wall meets the ceiling
I recall reading here that the wall paint should be used to fill in the tiny curve between the wall and ceiling paints. How strange would it look if the ceiling paint came down and filled that curve?
I do not have a steady enough hand to do this freehand - I have to use painter's tape. And even applying painter's tape on the ceiling in a straight line is difficult. I can, however, get a straight line of tape on the wall. I would let the wall paint cure before applying the tape and painting the ceiling.