One or two threads dont seem like much, but I just finished a Gods eye block that ask for a scant 1/4 seam I sewed a 1/4 seam and was short(not a 12'' block) but the block had over 60 pieces so the seam differences made a difference in size.I would think if one used the same pattern through out a quilt and same seam it would be ok.I was doing a sampler and didnt have a great out come.