Prism99

I would advise against washing the blocks before they are quilted. The muslin may shrink and distort in unexpected ways, especially since there is no way you can know what quality of muslin was used or whether it was prewashed before the embroidery. After quilting the batting helps keep the muslin from distorting (especially if they are closely quilted). Also, washing unfinished blocks can result in frayed edges.

You also need to be aware that many embroidery threads are not completely colorfast. This is another reason to wait until the quilt is finished before washing. When you do the first wash, you can use a washer that uses a ***lot*** of water to dilute any dye bleeds so that they do not settle into the muslin. I would also use Synthrapol and/or some color catchers in that first wash.

Edit: I should add that older embroidery threads are usually more colorfast than newer ones. This is because some of the chemicals that were effective in setting colors were eventually deemed too toxic and safer chemicals (but less effective at setting colors) have been substituted.