Please bear in mind that I have worked in quilt shops a long time and totally support that kind of business...
I would expect a quilt shop to take full responsibility for accurate measurements. It's just like any other business. How would you feel if the butcher or gas station or electric company shorted you "just a little bit"? What if the beef roast you bought had an unusual amount of fat or bone that left you with less usable meat? Would you see it as the buyer's responsibility to buy extra in order to compensate? I would probably take my business elsewhere.
We are strongly encouraged to support our local quilt store, and I heartily endorse that idea, but I also believe that they need to be professional in their business operations and make it worthwhile for us to spend our money there. If I am paying for 9" of fabric, I expect to get 9" of usable fabric. If I am buying 5 yards of fabric, I expect to get 180" of usable fabric.