I have seen patterns that can be done either way but if you do the top and bottom first you distort the visual length of the quilt. It increases the visual appearance of the width and shortens the visual appearance of the length. Mitered corners will keep that from happening. So, it up to you to decide what is important to you from a visual point of view. I like to cut lengthwise borders on the lengthwise grain of fabric and the top and bottom borders on the crosswise grain.