I really don't have any hard set rules regarding the backside of a quilt.....but....I do have a few preferences....LOL.
1. I prefer not to use stripes or an obvious directional print (difficult for me to keep straight)..... small prints or solids are ok.
2. Pieced backings are ok and since I quilt all mine on my Janome 6600, the direction of the seams don't matter.
3. Bobbin thread color is individual to each quilt. Some quilts will just scream at me to use a thread color that will stand out. Others need to blend.
4. Selvages always cut off, as they might shrink differently than the rest of the fabric.
5. Always prewash....I take no chances with bleeders.