the number of decimals for round of tax is set at the line 1430 in the Cart.php
$compute_precision = $configuration->get('_PS_PRICE_COMPUTE_PRECISION_');
strange thing is, that I don't have set it.
if I am not wrong, it means that in my case is $compute_precision set to 0 (decimal places)
therefore check, plz your value of '_PS_PRICE_COMPUTE_PRECISION_'
and if it is not set in ps_configuration, set it
BTW - I am sorry - I don't still understand to how many decimal places do you want to round