I was at one store that ripped, however they measured 2" past the yardage and ripper there. The local LQS I shop at will rip the extra wide backing, however they also go a bit longer to allow for the damaged edge.
I prewash all my fabric, so I rarely end up with something that will hang with the edges even. I always buy a bit extra than needed since I make mistakes - and I also love having scraps!
way back when I did garment sewing, it was normal that fabric stores would give an extra inch or two to allow for the fabric being wound on the bolt off grain, since straightening it was required before cutting the pieces. I was surprised when I stated buying quilting fabric, and saw them cut it as exact to the measure as possible.