R12 iProc EBTax, not sure what the point of enabling is per Note 552107.1

Per the above note, it is possible to enable EBTax functionality in iProc.
However, per Note 560339.1 and the document E-Business Tax - A Purchasing Perspective (from Note 464243.1) (p5 and P34)

"User updated Tax Code on the requisition lines will no longer be carried over to the PO document during autocreate. The Tax Classification will always be defaulted on the PO document" - p5

"During AutoCreate the tax will be redefaulted on the purchase order based on the determining attributes of the purchase order. Tax overridden on the requisition will no longer be passed to the purchase order, as was the functionality prior to release 12" - p34