Exchange Rates

When we run the period end accrual for foreign currency PO's it uses the exchange rate from the PO (I believe this is the daily rate when created).  When entering an invoice it will use the daily rate of the invoice date on the invoice.  The PO rate isn't the most accurate for accruing.  Using either the daily rate of the receipt date or the last day of the period being accrued would be more accurate.  Are there any options for getting a different rate or can anyone provide a good reason for Oracle using the PO rate?