I do not take my quilt out of the machine at the corners. Once you understand what is going on when you fold back, slide up and fold down, you will be able to do that with the needle in the fabric at the corner. My seams are 3/8 inch, so I stop at 3/8 inch from the corner with needle down. Turn the quilt so you can sew on the diagonal into the corner right to the edge of the quilt. Usually about 3 or 4 stitches will do it. Stop with needle down. Turn the quilt so you can sew down the next side. Pinch the binding to make that mitered corner. Lay the pinched fold down so that it looks like it would if you had done all the folds as above. Now lift the needle, slide the quilt over so you are ready to sew down the next side, and away you go. There is just 3/8 inch of loose threads as you moved over to the seam line. No need to cut those off. They can just stay there.
Happy binding!