Making Tax classificaion code mandatory in AP Inv WB or generating a report which states Invoices w

Hi All, I have created tax codes and they are working fine when selected in AP Invoice.
Client has lot of Invoices to enter in AP and at times forgets to enter tax classification code
Can i make this filed mandatory- If so can it be made OU specific.

If the above is not possible is it possible to get a report which will list down all Invoices with TAX CODE as NULL

Any pointers gr8ly appreciated

Thanks,
Satish