I use good grade muslin on all my quilts, and since I do not wash the fab, I do not wash the muslin. But, I must add I do not use starch in construction either. This way, when the quilt is finally washed (probably a year or two down the road) it will all crinkle evenly. If I use a color fab that I think "might" run, I will snip a piece, put in a cup of very hot water, if it bleeds, I don't use it. This is done before piecing begins......so far so good.....15+yrs.....but we all have our own way of doing things...