I don't think the answer is just to cut bigger pieces. The solution, of course, is for the store to sell straight cuts. Unfortunately, as the seller who asked this question explained, the fabric is not wound straight on the bolt.
She can take the bolt of Moda, Benartex, Hoffman or any other quality fabric off the shelf, line it up perfectly on the mat and cut a precise 9" at a straight 90 degree angle to the selvedges, and it may even appear straight when it's unfolded. But when you examine the cloth, especially if you wash it, you will see that it is NOT straight.
One solution is for the seller to tear a strip off the beginning of each bolt and then try to refold and rewrap the fabric so it will be straight for subsequent cuttings. It's virtually impossible to do, however.
This is why I prefer my fabric torn to measure instead of cut. I don't want fabric that LOOKS straight -- I want fabric that IS straight. And if it can't be made to BE straight, I don't want to use it in a good quilt. I might use it for tablerunners, coaster, placemats, or other crafty items.