I pre wash all of my fabrics. If is going to shrink or fade or bleed or otherwise misbehave I want to know before I start making the quilt. I think most, if not all would agree to pre wash flannel as it usually shrinks quite a bit. I learned that one the hard way in high school when I made a cute flannel shirt without pre washing it & it shrunk like crazy. I also managed a few JoAnn Fabrics way back when and had some allergy problems caused by the sizing and things used in processing fabrics. So pre washing was always my choice. I'm a steamer too- I just always have, never knew not to!
To each their own though. There are many accomplished quilters who don't pre wash their fabric and have no problems.