If pre-washed, the squares will shrink and will no longer be 10", and will no longer be square, as fabric shrinks unevenly.
I've never understood how shrinkage cannot lead to problems, when the shrinking happens after fabric is stitched into a block. If you pre-washed a 10" square before using, and had planned to use it in a block that called for a 10" square, you would no longer be able to use it in that block, as it would no longer measure 10" square. Yet it works out okay when shrinkage happens after stitching? Is this because the quilting stitches hold everything in place?