To the best of my knowledge, and as you've indicated, ASTM A6, which covers mill requirements, dimensional tolerances on the cross section, and the type of mill conditioning that's allowed, doesn't address the radius at the angle toes; nor does D1.1 or the AISC. Evidently the effect of the radius must not be significant enough for consideration when determining the weld at the toe, otherwise I would think that it would have already have been addressed, though, I've never really thought about questioning it. I was told by someone with Nucor Berkeley, of SC, that the radius at the toe is shaped by the edger, which will vary between angle sizes and thicknesses as to how "sharp" it is, and it will also vary between rolling mills.