i'm glad you posted this in an understandable way. i do queens about 96-98" square and i always buy 9 yards. those extra inches are just fine with me. on the la frame they give me some room on the sides to check tension and stitch length. on the top and bottom there's plenty for attaching to the leaders. when i take it off the frame and trim, the trimmings are always useful enough to keep for stash, which i don't mind. if i buy 108" backing fabric i buy 3 yards.
i always make the store unroll the entire 9 yards to check and make sure there are no misprints, slubs, or other defects before i get it home. sometimes you find that at the factory they have put two pieces together with those big white stitches in order to fill up the bolt. if it falls in exactly the right spot, i'll take it that way, but i expect a good discount. if it appears in a place that makes me have to piece across the width, i refuse it. it makes the owner very unhappy, but it would make me very unhappy to buy it. also, if the store only has one 8-yard bolt, i have them order another bolt before they cut the first two lengths. i would then have 2 lengths from one bolt and 1 length from the second bolt.