I ask my customers to make their backing & batting 8" wider & 8" longer than the top. Often backings need to be properly squared up. Plus, the excess allows for distortions that can take place during the quilting process, and to give some extra leeway for squaring up the entire sandwich when quilting is complete.
Many of my customers have me trim their quilts to within an inch and allow me to donate the left over pieces for charity quilts. Others like to keep the excess & use it in scrappy quilts. Little to no fabric ever goes to waste :)