Piecing the back adds a lot more variety to the quilt, almost like having a reversible quilt.
If you want to use a single fabric for your backs, especially for baby quilts, I highly recommend the diagonal seam method. It requires the minimum amount of your single fabric. Baby quilts are the ideal practice place for trying this method out. Here is a good calculator:
https://jinnybeyer.com/wp-content/up...-Worksheet.pdf