The rounding error is actually twice that, because what we see is a stem, i.e. the black distance between two lines. Each point on either side of the stem has up to 0.5 units rounding error, so the stem can be up to 1 unit off. (left point rounds down and right point rounds up = +1 unit, left point rounds up and right point rounds down = -1 unit)
Still it's hard if not impossible to see, and by increasing the upm in TT conversion it gets even more negligible.
But I understand that designers want to be in control, I myself checked all the ink trap lenghts in my interpolated instances