Do you stay stitch your curved pieces before sewing them? It's an extra step, but it prevents the fabric from stretching on the bias as you work with it. Stay stitching helps preserve the size and shape of the cut piece.
I tried to skip this on a dress I made for Sarah, and learned the hard way that it's worth the extra minute or two, LOL. It takes a whole lot less time to run a stitch than it does to rip out an entire seam - or cut a whole new piece when you rip the fabric!
Oh, and you can see what the others are talking about here:
How to Clip Curves on a Seam Allowance