Just follow the ceiling. It's especially important to keep the bottom edge the same distance from the ceiling. If you just nail it up but guesswork it changes the angle of the miter in the corners. I would suggest mitering a joint and fit it together to see what is the correct distance the bottom should be from the ceiling.
This must be some very small/light crown if it bends to follow the ceiling Also if there are many visible gaps you must have a wavy ceiling.
I have only done one house, but all I did was keep the crown in touch with the ceiling. Corners came out fine. Very few places where I felt it necessary to use any gap filler between crown and ceiling.
If it is slight variations follow the ceiling. Is the crown painted or stained? If painted fill any slight gaps with caulk. If stained scribe to the ceiling andcarve out the top of the crown ( belt sander or planer) to match the curvature of the ceiling.
A forum community dedicated to professional woodworkers and enthusiasts. Come join the discussion about shop safety, wood, carpentry, lumber, finishing, tools, machinery, woodworking related topics, styles, scales, reviews, accessories, classifieds, and more!