Sometimes a quilt binding just calls for bias. Not always, though, and I usually do use straight grain binding. There is really only one 'rule', as far as I know: Always use bias binding when the quilt has curved edges. A scalloped edge, for example, or curved corners. But there are also important structural reasons to use bias binding, even when a quilt's edges are straight.
Fabric has three named grains: straight, cross and bias. The straight runs parallel to the selvage edge. Cross runs across the fabric from selvage to selvage. The bias grain is on a 45º angle from the selvage edge. There are lots of illustrations to help us visualize this. Once you can envision the difference between bias threads and straight grain threads, it's easy to imagine how those threads fall on the edges of our quilts. Have you ever seen an antique quilt and noticed that the binding was worn much more than the interior of the quilt? It's probably because straight grain binding was used. Straight grain binding only has one or two lengthwise threads laying on the fold of the binding as the outer edge. Crosswise threads weave over and under those two threads, but those aren't really adding much strength. What happens when the lengthwise threads get weak? mmhhmmmmmm.
Bias binding, however, has multiple threads woven together across the fold. Many bias threads are interlaced together, adding strength to the edge of the quilt. And, as an added bonus, it has stretch, which is why it works well on curved edges!
Bias binding is much easier and faster to make than straight grain binding! Here is a great video tutorial, from McCall's Quilting: How to Make Continuous Bias Binding.
Oh yes, there may be one more 'rule' that is my own: When using a stripe fabric, always make it bias because it twirls and twirls around the edge of the quilt, making a very fun 'candy cane' effect (as in the photo above)!
Thanks for visiting! Happy sewing and God bless, Maxie