ROUNDING TAX AMOUNT

Hi

We are using SAP 8.81 PL5 South African Localization. Currently we have set currencies to round to Ten. On the marketing documents, AR Invoice, rounding to Ten on invoice value is working fine but unfortunately the rounding is not affecting the tax amount computed. As such, the effective tax rate is wrong, lower or higher depending on whether rounding is + or _ respectively. Please help me on how i can have both tax and invoice value rounded simultaneously.

Regards