I usually use bias binding because it can help me pull "ruffled" border edges into shape - I just apply a little traction to the binding as I'm sewing a problem area and the edge lies flat when the binding is sewn to the back. I've also heard that it wears better and that makes sense.
A few times, the only suitable fabric for binding is in the strips trimmed from the quilt after quilting, so I use the lengthwise and cross-grain strips to make straight bindings. Really, it's up to you - whatever you like to do.