I somehow forgot to add the essential part of my comment. Sorry, here it is:

If someone prefers a narrow tab width and breaks lines based on that, then someone who prefers a wide tab width will get overfull lines:

    20 column limit, 2-column tabs:
    if (a==b) {         |
      if (b==c) {       |
        some_function();|

    20 column limit, 4-column tabs:
    if (a==b) {         |
        if (b==c) {     |
            some_functio| <- overfull line

Either way, the project has to set some sort of official tab width. Or at least, a maximum tab width.