I usually prewash. The reason being is that I have had a couple of problems myself and seen others with much bigger problems from not prewashing.
If something bad is going to happen with the fabric, I would much prefer it happened before I put many many hours into the quilt.
If I am using cotton batting I preshrink it also. I do not want to give a quilt to someone and have them think they did something wrong when they wash it and it is puckery. I am one of those that does not like that look.