I sew my borders together first and then remeasure and attach them to the quilt. I find that if I sew on one border and then another to that one and so on, my borders seem to grow. Making the strata with all the borders sewn together, ironing and measuring to get the same lengths help me to keep the waves out of my borders. I am sure everyone has their favorite method that works for them. As for piecing the borders, I would tend to stagger the seams unless you can work the seams into a design feature. Sometimes if I don't have enough length to go the whole length of my quilt, I place an extra block in the border. This solves the seam issue and continues the design out to the edge of the quilt which I some times like better.