My thoughts is that it shouldn't really matter. I assume here you are talking about the balance taps.
The negative will be attached to a positive of the next cell in the series string. Thus it should be at the same potential difference (voltage). I would hope that your wiring connections between cells are thick enough (and with a low enough resistance) and short as they can be, to make the difference negligible. At some point, if you have OCD, it's gonna mess with you that you have one on the negative and the rest on the positive or one the highest cell positive and the rest on the negative. Perhaps if you put the cell tap in the middle of the connection that would be the most symmetric and be least offensive to those who can be without symmetry.
Hope that all made sense. My personal choice would be use the cell terminal connections if you have them like on big prismatic cell. That's only cause it is easier to make proper contacts.
