Question on Payables invoice match option

I have a client that is considering changing the Invoice Match Option from the current setting (Match to Purchase Order) to Match to Receipt in their system.  They have external systems that they use for all of their direct purchasing and receiving.  PO's and Receipts are then interfaced to Oracle where the invoice is entered, matched and paid centrally in Oracle.  To complicate things further, foreign currency is involved in many of their transactions (Canada Operating Unit for now - but will be world wide).  They want to change the match option to receipt to better reflect the timing of