How are you doin the corners? If you are mitering them, then you need to over cut them the width of the border. You can also do corner blocks so it doesn't matter which you do first. If you are just sewing straight ends over the other boder end, then it depends on your preference. Sometimes if the quilt top has strong vertical elements I like to do the top and bottom first and do the side borders last to reinforce the vertical look. If I am going to be short of fabric, I do the sides first and then the top and bottom as the pieces usually use less fabric length.