pre-washing is a personal preference. I only pre-wash those that I suspect will bleed(the colors run out of the fabric because the dye wasn't set right). These are mostly batiks and many different reds. there are some purple and blue bleeders but not many. if you are worried you can always cut a section to test to be sure.
Another suggestion that I have is regardless if you pre-wash or not is to get starch and practically soak your fabric in it. I have recently started to do that and it has made my cutting and piecing sooooo much easier.