You will notice that my test fabrics have some shapes fused on them. This is so that I can test my applique stitches before using machine applique on my actual project.
There are also fabrics that I don't use very often, such as organza, satins, and linen. This is when it is most important to have some testing fabrics ready. If the pattern doesn't leave any extra for testing, it's well worth it to buy an extra eight of a yard for practicing or testing.