If I am making a very nice quilt, say for a wedding gift, I will buy enough fabric to be able to cut the borders from the length of the fabric, so that I don't have to piece them. If I am making a utility quilt, say for a grandchild to drag around, I will cut the borders WOF, and piece them.
Buying enough fabric to cut the borders w/o piecing adds to my stash, so it's a good thing.