I love making flannel quilts. So do all of my loved ones who get them for presents.
If you want them to be flat, prewash. If you want puckers around quilting, don't prewash.
My first flannel quilt I used JoAnn's. Never again. For a baby quilt that will be outgrown or washed and washed perhaps would be OK.
Expensive as good flannel can be, just like any other quilt you wish to last, it is worth the investment.
I find simpler patterns to showcase the prints and colors are easier to deal with though I love doing raggy quilts with applique.
My family doesn't want their regular quilts during the winter and the flannel bed quilts are brought in to watch TV or wrap themselves while playing board games.
My grand nieces and nephews call me to tell me how tall they are to get new flannel or raggy quilts. I have to do four full size this spring to be ready for Christmas.
I would say don't confine yourself to patterns just for flannel but check out a few books from library so you learn about different fascets of working with this fabric.
If I knew how to post pictures I would do so to show a few examples. Oh, I love to quilt designs in flannel as they show
up so well if you don't prewash.
Good Luck