The modern flannel prints seem to stay nice and bright and the fuzzy often gets even better after washing.
I'm such a stickler on prewashing I just can't not prewash but I don't use starch, we all have our way of doing things. I recently backed a donation quilt all cotton top with flannel for the first time in a long time, I did wash everything first and again after quilting with no issues. Still not my favorite to work with but it makes a nice cuddle.
So my advise is to always prewash. It's going to get washed sometime so better that all the give and take happens before it made into something else. Also I generally figure that if anything does go wrong, I'd rather it happen on my watch.