I've been washing, starching and ironing all of my fabrics lately. Unless I'm making a rag quilt, then no pre-washing.I have a high efficiency machine with a delicate cycle that really doesn't agitate the fabrics at all and so I don't have the thread mess. I pre wash to see how the fabrics behave when washed (look for bleeding etc) and so they will be pre-shrunk. I don't think it is an absolute must though. Just whatever you prefer. Unless you get some stinky fabric :)