Comment 11 for bug 868839

may be you want to try
http://bazaar.launchpad.net/~c2c/c2c-rd-addons/trunk/files/head:/c2c_account_tax_rounding/

Default tax calculation is per line including rounding of tax amount per line.
This module turns off rounding per line and per tax, hence simulation vertical calculation
standard: tax = sum(round(net*tax)) (~decimal)
this: tax = sum(net*tax) (~float)
WARNING - in extremely rare cases this may also produce another result than
sum(net)*tax
which is the desired outcome.
Set "Rounding Precision" in Tax definiton.