Problem with rounding of transport in sales order

I am looking at problem with sales order where transport is entered as condition on header level and is allocated by the system to the line items. I am not sure how the system does the calculation but in each line item the net value after transport multiplied by the quantity has difference of couple of cents with condition value displayed. I understand that this is due to the rounding as the system displays the value up to the second decimal point, whereas it does calculate it with more details. But this is a big problem because the invoice amounts doesn't match the down payment invoice. Is there a way to change the calculation of rounding differences?