You can do a border in many different ways. I think it is just a matter of personal choice. I usually do the sides then the top. I have only mitered one quilt and that was because of the fabric I used and I wanted to try the mitered process. Just be sure you do the border accurately so your quilt is square and easy to quilt. Many people do not know the proper way to measure for a border. Don't just pin it to the side and whack off the extra. There are many tutorials on how to measure a border. Be sure you understand the process no matter if you piece your border, use one strip or miter the corners. The best advise I can give you.