In OpenERP, the Tax calculation rounding method in e.g., SO/PO/INV have 2 rounding method.
This can be setup in menu Setting > Configuration > Accounting
Round globally, where rounding per line in 5 digits and then round again in 2 digit globally.
Round per line, where rounding per line in 2 digits.
I would recommend Rounding globally always. In some very rare case, Round per line can give some digit problem as following,