If you join your pieces at an angle, whether it is for borders, sashing, or binding, the seam is less noticible, especially if the fabric has a design.
Most longarmers will ask that your batting and backing be 3-4" wider and longer than your quilt top, so take this into account when you make your backing. One of the longarmers I used preferred to use her batting, and the price was comparable to what I paid for batting, and another preferred her clients to bring their own. Also, if you want her to use black batting, let her know. She may not have any, so you will have to supply it.