I prewashed the lengths I have worked with (bought a bunch of the "not kilt selvedge" run) and am glad of it. The blue rubbed off so easily as I handled it just in cutting, prior to washing, that I'd have had blue hands and worries about it discoloring other surfaces as I worked with it.

Put it through the dryer as well, can't remember if I tossed in a dryer sheet -- probably did, it's a habit. No ill effects although I can't say if the run I got is T-coated or not.