I never prewash unless it's flannel or maybe a red. With the red I cut a small piece and put it in hot water to see if it'll fade. If it doesn't, I don't wash.
I like the look of old quilts that kind of puckered because of the cotton batting they used so I just let the fabric do it's thing.
There are alot of people who will tell you the exact opposite. I think it all comes down to personal preference.