One of the advantages of using a small bent needle file to cut the muller in is that you can make the width of the muller any size you want. I can't do that with a concave cutter. Those are one size fits all.

A quick internet search titled "what is a mullered border" turned up many relevant hits including one from none other than Brian Dudley (from his web site, it appears) on the subject. It was an interesting and informative read on Parker checkering with mullered borders and includes clear, detailed photos.