THe question is: when I add borders (or when I add multiple borders!), do I miter them at the corners? The answer depends on the quilt, the complexity involved, and the fabric. I am the first to admit that I take short cuts when they serve me well, and so if I can just sew strips onto each side of a quilt and call those a border well-done, I don't have a problem with that. It makes life simple and efficient... and they're easier to do, too!
On the other hand, sometimes I may be adding multiple borders at the same time (as I did on the last portion of the July instructional video). In those cases, it is easier to sew together the many floaters that make up the rows of borders first... and then add them as a set to the quilt top. In that case, butting a set of borders onto each side would not give nice finished corners; mitered borders would be called for. I happen to like mitered borders; I think they give a really nice "finish" to a quilt.
Another thing to consider, when deciding whether to miter or not, is the fabric itself. Sometimes a fabric that is butted against another (i.e., not mitered) does not give a nice result - particularly with border prints or stripes; butting them against each other results in a disruption of the pattern within the fabric. In this case, if the fabric itself tells me that it needs to be mitered, I listen!
I hope that answers the question!
Sue Garman