You will find many many threads on this question if you do a search with just as many opinions. I myself am a prewasher - learned the hard way with shrinkage. The fabric comes home from the store and goes in the laundry pile. If you take it right out of the dryer you can avoid all the wrinkles. I prewash not only due to fear of colors running, but mainly due to shrinkage. Different brands will shrink differently and when sewn together you can have a mess.
For the Round Robins we have to prewash, and I was discussing this with my LQS owner when I was buying my fabric for round 4. She won a beautiful quilt in a raffle from a shop hop (you know it was quilt shop material) and long story short had to wash it. It has lots of little pieces and all different colors. She threw in five color catchers. When she removed the quilt from the wash many of the colors had ran. Pulled it from the dryer and the smaller pieces had shrank and with the quilting on them were unrecognizable. The was ruined! Had the fabric been prewashed she would still have the quilt.