Bree123

Woolies flannel is 100% cotton. There are 2 camps as to how to work with cotton flannel (called "flannelette in places outside the US").
1) Pre-Wash & dry in warm/hot temps to cause the fibers to shrink up as much as possible, creating a dense fabric that doesn't really have anywhere to shrink because all the gaps have been filled in by shrinking in the laundry process with heat & agitation. The flannel-making process damages the cotton & exposing it to that much heat & agitation weakens the fibers further so this is not recommended for heirloom quilts, but it will produce a nice utility quilt that can endure harsh laundry conditions with little to no further shrinkage post-quilting. I use a Microtex needle to quilt such flannel because of it's density. It's a bit of a tough call because on occasion it does snap one of the fabric's fibers, but I find I usually have the best luck with it creating a nice stitch without visible holes around my thread.
2) Pre-wash/dry only in cold water with very limited agitation, line dry or tumble dry on low heat until mostly dry & then finish with a dry iron. This process will better preserve the integrity of the fabric, which is both a plus and a minus. On the downside, that means that the quilt may well shrink if it's later washed in warm water or dried on medium heat, requiring mending/restoration to be able to get it back to a decent condition. On the upside, though, you have stronger cotton fibers that will quilt beautifully with a thicker thread like a 28 or 40wt. I use a Quilting needle for that because the fiber is a looser weave so the slightly thicker needle isn't a problem & the rounded tip helps prevent the fabric's weak fibers from snapping. The other upside is that fabrics washed in cold water & dried on low heat are far less likely to bleed than those laundered in warm/hot temps. Nothing in life is guaranteed, but other than custom dyed flannels or some rare super saturated flannel solid, I find that modern fabrics in flannel don't cause me problems when washed in cold water (which is how quilts were safely washed for generations before electric washing machines were invented). If you are working with either of those 2 types of flannels, I'd definitely at least swish them around in the sink with a Color Catcher for 20 minutes and let them air dry.

If you are only working with commercial print flannels & no other fabrics, it's a risk, but you could always give it a try without any pre-laundering. Maybe make a small sampler like a throw pillow cover without any washing, lightly quilt & then wash it on cold in the gentlest cycle you have & maybe fluff a bit in the dryer before finishing it on a clothesline. I personally wash all my pre-cuts due to a severe allergy to sizing, so I haven't tried it myself, but I really don't find that when I wash cotton flannel in cold water & dry on low heat that it shrinks more than 1"/yd even when I stick with the cheapy stuff from JAF. If you give it a try, let us know how your experiment turns out!