Yes, the nacelles do taper but the nacelles do not have--or should not have--a level edge; the top edge tapers downward from the dome and the lower edge tapers upward, so it's the horizontal centerline/axis of the nacelle that has to be level, not the bottom edge (see schematic, above post). If the bottom nacelle edge is level/parallel with the saucer, then the nacelle is sagging. It sounds nit-picky but it makes a big difference visually.