I dont prewash. And so far, no problems! One exception is when I am doing block swaps, the rules say pre-wash. I have even backed my quilts with flannel, not prewashed. They shrink a bit more than plain cotton, but I didn't have a particular size the quilts needed to be. So it was all good!@
I love the crinkly look the quilts get when they are washed after all the work is done.