I have had flannel shrink that much so I always pre-wash it.
Flannel excluded, I find just pressing the backing with a hot iron shrinks it enough for me to get it squared and loaded on the frame. I like the wrinkly look so my top isn't usually pre-washed either.
I have had some really thin & loosely woven fabrics shrivel up when I press them out so I know right away not to use them until they have been pre-washed. Muslin especially has very different thread counts. You can't always go be the "feel", either because sizing and starch makes the thin fabric feel thicker. Holding it up to the light is the only way I know to check it before getting in the cutting line.