I've seen the backing done in many many ways. Big blocks of fabric from the front, half and half, in thirds. I usually end up doing the back in different sizes.......just cause I want them to be be used and loved. If you give it a "look" to the back, they'll love it that much more.
Don't be afraid to TRY anything, if it's pleasing to your eye, it's going to be wonderful. If by cutting on the bias it frays then use your zigzag. No quilt police will ever say it's wrong. I promise.
Merry Christmas,
Sharon