Most of the time when I go to JoAnn's I don't think the person at the cutting table knows much about sewing. It's just a job for them. Local quilt shops hire people generally who are quilters and love to help people.
I piece my bindings all the time. Sometimes I use bias binding, sometimes straight grain but I always piece them on the bias so the seam is nearly invisible. When you consider once the binding is in place you have less than 1/2" visible from either side and the seam is slanted because you've sewn it on the bias what difference does it make that you've been thrifty with your fabric and pieced it. The only thing I watch out for is to not have a seam on a corner but a quick audition of the binding on the quilt will give you a place to start so that doesn't happen. For that matter I also piece my borders. Again I sew the seam on the bias because it hides it more. For most prints and dark fabrics you have to really search for the seam.