I've used various round objects over the years to round the corners of my quilts. I use bias binding if I am going to go around curves. The last quilt with rounded corners, I cheated. I used fabric cut straight of grain for the straight part of the quilt and sewed about a 12 inch bias sections on as I came almost to the corners. I sewed the bias section around the curve and then re-attached the straight binding and continued. Of course I ended up with 8 join spots but the fabric was a dark pattern it was hardly noticeable. If the quilt is not going into competition, do what works for you.