I only pinned at the centers of the curves. I used those long angle pointed tweezer like things to hold the beginning and ends. Like this one from connecting threads:
http://www.connectingthreads.com/tools/Sewing_Machine_Tweezer__D82294.html
I have made several curved quilts without all that pinning and had no problems. I put a finger between the top and bottom curve so I can meet the edges appropriately. You an pin the beginning and end if you want, but I found the tweezers worked better for me.