I have seen it done many ways. The way you will know for sure on your project would be to test it out. I really most often will do it the way thimblebug is showing in her cute block. Sometimes I do like to use the thread to be an accent with a darker thread color to "shadow" The piece edges. I used to use black on the old-time sunbonnets but I changed to a white to kind of update the block to a modern feel. It really is maker's choice and what looks good to you. Julie.