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cut off the selvage, because it is woven tightly and could skew your quilt slightly or not have the same amount of give as the rest of the quilt.
I have only been piece quilting forabout 2 yrs now, but I love plain muslin for the backs of my quilts. I use thread or pearl cotton (for tying) to match the top. Nobody has complained yet. smile.....Quilts for little folks I do use a matching print .