Because they are jelly rolls, I don't think I would prewash--could be lots of fraying. But if you decide to, don't put them in the machine, instead fill a basin with as hot a water as you can stand, (you don't really need to WASH them to get them to shrink), soak a few like coloured at a time, squeeze out as much water as you can and hang dry over a towel rack or something similar. They will have to be ironed too of course. This will also allow you to see if any are going to bleed excessively. If any do, you can soak in a mixture of water, vinegar and a bit of salt, then rinse. That should inhibit the bleeding.