I've used cotton men's shirts from the Salvation Army and they seemed to hold up OK. If you are concerned make a mini quilt or placemat and launder it. I would definately pre wash any new fabric. The older stuff is not going to shrink so you're ok there. Given your excellent work on these blocks, you should be able to handle assembling the quilt on point. Finally, I like the dark blue sashing. I like how it makes the secondary squares with the cornerstones.