I would have washed the muslin first but I'm a pre-washer and I don't like my fabric to shrink. As for sashing--it's all up to you. Would it look better with or without it? Trying to make the quilt bigger? Using one color sashing as a way to tie all the scraps together? I would put my blocks up on the wall or lay them out on the floor and let them "talk" to me.