No. The dish is concave..meaning that the deepest part is the centre. If you were to put a flat plane on the dish, only parts of the plane would touch the curve (the corners of the ends of the plane because they are the furthest away from the centre of the dish. This assumes that the length of the plane is greater than the width).
Similarly, with your sides in the mold, you gradually sand on the dish, more material is removed from each end before the midline (waist) would touch the sandpaper. If you were to take the sides apart and stretch them out, the waist area would be thicker than the ends (i.e. More would have been sanded from the ends than the waist). Having a taper saves you from excess sanding.