Okay I am going to go against the norm. If it had a cotton top and flannel back I would prewash. If the baby quilt is going to be all flannel and you don't have a fabric that is a bleeder ( check with a small piece) I wouldn't pre-wash.
If you wash before you might get pills or very wrinkled fabric to work with.Do you want to know if the flannel is going to pill before using it? That's your call but I like a nice new looking flannel quilt for gifting and then include washing instructions with it.