backness 0 or -

[milbert3]

Hi,
I am wondering what the systematicity is in the coding for “back” for the following sounds. In many sounds marked with a "0" for backness, would a "-" not fit better? (Except the velar ones, there it would be "+"?)

• Bilabial trill (ʙ) and alveolar trill (r): 0 back.

• alveolar (ɾ, ɾ̝), alveolar-lateral (ɺ) and retroflex tap (ɽ): 0 back.
  --

• labiodental and velar approximants (ʋ, ɰ): 0 back.

• labiodental nasal (ɱ): 0 back.

• other nasal and labiodental sounds are + or - back.
  --

• retroflex and velar lateral approximants (ɭ, ʟ): 0 back.

• lateral fricatives (ɬ, ɮ): 0 back.

• alveolar and palatal lateral approximants (l, ʎ): - back.
  --

• labiodental nasal (ɱ): 0 back.

• other nasal and labiodental sounds are + or - back.
  --

• velopalatal fricative (ɧ): 0 back.
  --

• uvular trill (ʀ): + back.

• alveolar approximant (ɹ): + back.

• retroflex and palatal approximants (ɻ, j): - back.

Thank you.