I'm not sure about what they want for shows, but I only use straight. Really the only time bias would have an advantage in real life would be for a scalloped or curved border/edge of a quilt. The binding will bend a LOT easier on curves when cut on bias. This of course is just my opinion, but I think a lot of people will agree. BTW all my quilts are wallered on by cats, me, the hubby and are washed a lot and I havent had a problem yet.