I never prewash, however with any fabric that I think may be a bleeder, I always use a dye catcher when I wash the finished project. If its a large piece, I will cut a small square and test it in hot water and soap. I took a class on fabric with Harriet Hargrave years ago and is what she recommended. I have not had a problem. One reason she recommended not washing is that the coatings the fabrics apply during manufacturing help to protect the fabric. If you are person who only buys enough to do one project at a time, that may not be an issue, but I am a bigger fabric collector than cutter!
For block swaps that want washed blocks sent, I assemble them, then wash as she recommended.