I prewash FQ. I've done tests with both FQ and yardage and found some of the shrinkage is enough that my finished project would have been terribly wonky. I envy those that don't and have not had problems. But since I mostly hand quilt and it takes me 6+ months to finish a quilt I don't want to risk any odd shrinkage. Also, I know the quilts I make will end up in the wash and the dryer with no special handling so pre-washing is a must.
If I were making something for display or knew that it would only be gently washed and laid out to dry I wouldn't worry about pre-washing.