Fit the crown molding to the ceiling as best you can. After insulation, use painter's caulk to fill the small void between the top of the crown and the ceiling. Professionals do this all the time. It's so subtle, you'll never notice that the ceiling is slightly elevated from the top of the crown.
To guarantee a nice clean line of caulk along the top of the crown molding, take the extra time to tape it off with some quick-release painter's tape. Immediately after smoothing the caulk, remove the tape and you'll have a clean straight edge.