It's a personal choice decision. I use bias only on curves and scallops or for a special effect (stripes/plaid). On all other quilts I do straight of grain because it's easier and my quilt edge is straighter. I don't know if judges actually study the edge to check for bias threads. I do know they look for straight edges, good mitre corners (with the mitres sewn) and good, even, invisible hand stitching on the back.