Quite a while back I bought several 10-yard bolts of solid colored flannel from JoAnn's when they were having their Black Friday sale. (It was $10 per bolt - who could resist?) I decided to use some of the cream-colored flannel for the back of a comfort quilt. As always, I washed the flannel first. It felt yummy, thick, very nice - but when it was washed it pilled quite a bit. I didn't want that on the back of the quilt, so I used the flannel as batting instead. Another bolt, that I used earlier, didn't pill at all.
So... another reason to wash flannel is to see if it's going to pill.