A lot of the
crescent shawls I've knit require a really sturdy bindoff edge so you can block it out completely without losing the curved shape of the entire shawl. I created the following video to help with this technique:
Finish Your Knitting with a Sturdy Bindoff - the P2tog Bindoff Method
What if you are making a top down shawl? Then you would want a
stretchy bindoff!