Neither is right. It is a preference. I PREFER sewing to the front and hand sewing to the back. This is because my old machine does a poor job of it. Now that I have a better machine, I'm going to try machine sewing it and then you do attach to the back first and then sew on the front.
Keep in mind that if you are showing your quilt in a quilt show, make the effort to sew to the front and hand sew to the back. A lot of judges will take off for machine sewn binding because it's not 'what is done' OR because most people really do not do a superior job of it.