I sew lots of curves. One piece is concave, and one is convex. I fold the concave piece in half and snip the SA about 1/8 inch, then also in the mid-point on each side of the center (3 snips in all). Then take the convex piece and fold to find the center--to match up with snip on concave piece. Place a pin to hold centers together. After that, I line up (and pin) each end. Next, ease each half and pin. When I stitch, I don't lay the pieces flat, but rather lift and feed --The pieces are already "curved" with the pins. Hope this is clear as mud!
(The last pic shows my snipping)