In the instance of a concave curve where the fabric edge is shorter than the area it is folded to, the bias grainline will stretch, allowing you to manipulate the fabric. If it is a severe concave curve, you can clip the seam allowance in order to prevent puckering.

In the instance of a convex curve, the length of the fabric edge is longer than the area it is folding to, so you have to manipulate the fabric to make the lengths equal.

Here’s a little tip for finishing curved edges...

Single Fold Hem

(I recommend that you finish the edge with a serge or zig zag before starting. This demo is left unfinished so that you could see the stitches more clearly) 

1. 
   Sew a baste stitch 1/8” from the fabric edge, leaving a few inches of thread at one end
2. 
   Sew a second row of stitching at the intended seam allowance for the hem
3. 
   Pull on one of the thread tails from the stitch closest to the edge, drawing the fabric just enough so that the folded edge lays flat onto the fabric
4. 
   Distribute the gathers evenly and press
5. 
   
   Turn the fabric towards the wrong side, pressing along the second stitch line. This stitch acts as a folding guide and helps in achieving a crisp and even fold
6. 
   Distribute the gathers along the curve and press
7. 
   Topstitch the hem within the seam allowance

Double Fold Hem

1. 
   Sew a baste stitch ¼”away from the fabric edge. Sew a second row ½” away from the fabric edge
2. 
   Turning the fabric towards the wrong side, press along the first ¼” stitch line
3. 
   This helps to create a crisp even line to fold along the stitch
4. 
    Press again along the ½” stitch line.
5. 
   Topstitch along the hem within the seam allowance.  Feel free to remove the stitching along the folded edge after the hem is finished