Prism99

I should mention that there are a few times I do prewash.

If I am adding a bias binding, I highly recommend prewashing the fabric for shrinkage. Because the bias of the binding fabric is running unquilted along the edge of the quilt, it can shrink and distort the edges a lot. How do I know this? Don't ask! I had sewn the binding on with a dense decorative stitch, so it would have been difficult to remove. If I still had this quilt, I would re-wet it, block it to dry (stretching the bias binding to be as it should be), then quilt all over the binding so it would no longer have those long, unquilted lengths of bias fabric on the edge. I normally use straight-of-grain binding, and have never had a problem with unwashed bindings cut on the straight-of-grain. It's only bias bindings I will now prewash.

If I were sensitive to the chemicals in fabrics, I would definitely prewash.

If making a tied quilt, I would definitely prewash all of the fabrics. Ties do not bind fabric to batting continuously the way lines of quilting do, so fabric would still have a chance to shrink noticeably between ties.

As for bleeding, if I am suspicious about a fabric, I will test a square by dropping it into water, letting it soak, then laying it on a paper towel to dry. I don't worry about mild bleeding, but I don't want to have to deal with a really bad bleeder. A bad bleeder I would prewash and re-test to make sure it isn't going to keep bleeding. I wash my finished quilts at the laundromat in their largest front-loader with hot water and Synthrapol. This seems to take care of any bleeds for me.