Whenever I birth a potholder, place mat or similar items I always slip stitch the opening closed by hand after it is turned and before it is top stitched. You can do it without the stitches even showing by using a fine needle to get little stitches and using the fold to run your thread along. It makes the top stitching easier to do and overall just looks nicer I think. I recently gave some microwave bowls and potholders to a friend and she kept going around and around the edge to figure out how I made them as she couldn't find the area where I had turned them. I don't use my walking foot to top stitch the edge because it doesn't like it when I am stitching close to an edge and only a small part of the walking foot is on the fabric.