I'm a pre-washer for the reasons others have mentioned. I react to the chemicals in fabric, I want to catch bleeders before I put them with light colors, I want to know how the fabric holds up BEFORE I spend my time making something with it, I want to pre-shrink it.
Flannel will shrink a LOT - up to 7%, I have been told! Try measuring it before and after washing and you will be amazed. I wash it and dry it twice in very warm water and dryer.
All that being said, if you choose not to pre-wash, no-one will arrest you. It is your fabric, your quilt, so you get to decide the rules of the game!