I would pre-wash flannel, any flannel, no matter where you purchase it from. Most of my quilts go to children in foster care, so when I use flannel, I wash in hot water and dry in a hot dryer. I want to subject it to the worst possible conditions before I make the quilt. Why? There is no predicting the laundering habits of foster parents.
If you know the person you are making the quilt for, then my suggestion is still that you pre-wash flannel, but cold or warm water is fine, and dry on a medium setting.
Above all, have fun making the quilt!