i'm a half & halfer---half the time i pre-wash the other half i don't bother---it really depends on the fabric- if i have fabrics (like Kaffe's) that i simply am not sure about i simply test to see if they are going to bleed---simply spritz a small corner with water- rub it with a paper towel or piece of muslin- if color comes off you have a bleeder- better wash---if no color comes off- good chance you are safe.
i tend to prewash if the fabric is stiff- doesn't really feel good- or has a funky smell to it- or if it's a hand-dye- or batik with deep saturated colors-
i always test reds, deep blues/purples...if they are not bleeder's i don't worry about it-
and some flannels i pre-wash many i do not- just depends on what they are going to be used for- i use Benartex double sided flannels which are a bit more expensive but oh so worth the extra$$- i've never had one shrink enough to cause any problems- the only cottons i use that shrinkage may be an issue are homespuns-and i've made a few great home-spun quilts without pre-washing- since the batting and quilting also play a part in how much a fabric will shrink....anyway- when in doubt---test for color-fastness- other than that- 'don't sweat the small stuff'