If you have to join your backing fabric for any reason, the seam should be half an inch wide and pressed open. Always remove the selvages both in the seam and the outer edges as these do not stretch to the same degree as cut fabric and may cause problems keeping it flat while quilting resulting small tucks forming.
If you piece the entire backing for your quilt with different fabrics or into a pattern, it is not possible to guarantee that this piecing will be central on your quilt top. The reason for this is because, unlike quilting on a domestic sewing machine, we do not start in the centre, but on one edge working towards the opposite edge.
Something to bear in mind if piecing your backing with a lot of smaller pieces is that some of the junctions in the top may coincide with those in the backing. A very thick area will result which will be difficult to quilt over or if the stitch design goes around it, it may not lay very flat, an outcome that may not be very pleasing.