PO Tax amount is rounding off at Line Level

Hi,
When we are creating the PO and applying the tax, system is rounding off the tax amount at line level only.
Example :- PO line 1 Amount = 24,682.00 , Tax = 10.5% , Tax Amount = 2592.000
                  PO line 2 Amount = 25,398.00,  Tax = 10.5% , Tax Amount = 2667.000
                  Total Tax Amount = 5259.000


Our client wants that tax amount should not round off at line level. It must round off at Total Tax amount level -

Example :- PO line 1 Amount = 24,682.00 , Tax = 10.5% , Tax Amount = 2591.610
                  PO line 2 Amount = 25,398.00,  Tax = 10.5% , Tax Amount = 2666.790
                 Total Tax Amount = 5258.400 = 5258.000 Rupees

How can we make the changes in system to have this kind of calculation.

Regards,
Kapil