I don't think there is a rule about borders, but I pretty much add my side borders first and then the top and bottom. I measure through the middle of the quilt top before cutting my border, never measure the sides where you will attach the border. My borders lay nice and flat, no problems with them ever. I did read the Bonnie Hunter article about borders and that is pretty much how I learned in my quilting class. It works so I never changed my way of doing it.