Comment 13 for bug 748178

Hmm, that's why I didn't include it in my patch.

my thoughts
* it's not a very important column (I didn't try with module tax included)

* normaly invoice_line_tax_id would allow to compute tax by line (see move_line_get tax_amount)
 it just needs t be stored.
 this may return us back to the discussion about horizontal or vertical tax calculation and rounding and the need to fix this
see
http://bazaar.launchpad.net/~c2c/+junk/c2c_austria_dev/files/head:/c2c_account_tax_rounding/