I don't prewash. I started off prewashing because I was told you had to or all sorts of disasters would happen. Then a friend that owned a quilt shop pointed out to me they never washed any fabrics for any of the store samples. Besides if the dryer is what shrinks the fabric, my iron is waaaaay hotter than the dryer.
As to bleeding, if it's not set in the original fabric and you didn't do anything to it to set it in something that picks the excess dye up in the wash, it's still not set and will wash out eventually.
Simplest solution, don't wash any of it.