As a general rule I use bias binding on bed quilts, or quilts that will be getting a lot of use. It is more durable. If a thread or two breaks in bias binding, only a small part of the binding is affected. With straight binding - a broken thread or two can run the entire length of the quilt and really comprise it. Straight of grain binding, however, is good for smaller quilts, wall hangings and such as it will help the quilt keep it's shape and durability isn't as much of a concern.
Just my 2 cents.
Happy Quilting!
Dave