I usually cut my borders lenthwise too. Normally what I'd do is measure the length of the quilt in the center and cut the long sides first. Then I'd measure the center of the width and cut those. But if you are concerned about a seam showing, you may want to cut the top/bottom first and piece the sides if the quilt has a definite direction to it. If it's not directional, I don't think it really matters. You can always add another block of a different color or pieced block between the pieces to lengthen the borders. You should measure well and sketch it out to see what your options are.