If you are using all new flannel and it is the same kind/weight, you can probably get away with not washing it. If you are mixing it with some that has already been washed you will need to wash it as it will not shrink evenly. Personally, I always wash flannel because some of it shrinks more than others and I don't want to ruin all my work by having it shrink unevenly. I do not usually wash cottons unless I suspect it might bleed.