I often seam my borders and I try to place the seam in the center of the panel (it's a symmetry thing). Depending on my mood (and how much fabric I have) is whether I figure out how long the pieces have to be to accommodate the miter, or I just sew them on and cut off the excess.
The fact that your instructions tell you to cut five (5) lengths, indicates that you are expected to seam the long side.