I prewash everything and put a little white rag in with dark colours to test for running. For me, it's part of the bonding process with a new fabric and I love ironing it (NOT shirts though! LOL) The only fabric I don't prewash is for stack n whack as the sizing helps it to stack. But I test wash a scrap to see if it runs and if it did (hasn't yet!) I'd put a colourcatcher in when I washed the finished piece. I have had some fabrics run on me, not necessarily cheap ones and not only batiks. A side note - I don't use spray starch when ironing fabric that's going into the stash - apparently it can encourage silver fish. Not quite as bad as Candace's maggot story but bad enough!