when I am starting on a new type of a block or pattern, I have to make a test block due to one I'm making now. I needed to be very sure of which part of the braid was added to the correct side. So far so good.
On another level I like to make a practive block if I'm almost postive this is the fabric set I want to use together. When you are laying fabrics to look out, they can look mighty different after you make a block, then realized you don't care for it and you have cut your fabric in advance.