Impacted versions:

[9.0]
Tested in runbot http://174550-9-0-10dcec.runbot10.odoo.com/

Steps to reproduce:

Set the Tax Calculation Rounding Method to "Round Globally" 
Create a Sales Order with the following lines with 7% taxes

1) 5,095.20 
2) 523.20 
3) 80.64 
4) 54.12 
5) 182.00 
6) 186.20 
![image](https://cloud.githubusercontent.com/assets/16055718/19144073/309aa082-8bda-11e6-81d0-2461c4e9a467.png)

Create a Sales Invoice with the same lines with 7% taxes
![image](https://cloud.githubusercontent.com/assets/16055718/19144078/38048b58-8bda-11e6-9c2b-4b501cfa1e12.png)

Current behavior:

The total taxes in Sales Order are $477.48 

The total taxes in Sales Invoice are $477.50 

Expected behavior:

Both the total taxes in Sales Order and Sales Invoice should be 477.50 

Video/Screenshot link (optional):

if procurement.rule_id and procurement.rule_id.action == 'buy':
            return procurement.make_po()
        return super(ProcurementOrder, self)._run(procurement)
    @api.model
    def _check(self, procurement):
        if procurement.purchase_line_id:
            if not procurement.move_ids:
                return False
            return all(move.state == 'done' for move in procurement.move_ids)
        return super(ProcurementOrder, self)._check(procurement)
    @api.v8
    def _get_purchase_schedule_date(self):
        procurement_date_planned = datetime.strptime(self.date_planned, DEFAULT_SERVER_DATETIME_FORMAT)
        schedule_date = (procurement_date_planned - relativedelta(days=self.company_id.po_lead))
        return schedule_date
    @api.v7
    def _get_purchase_schedule_date(self, procurement):
        """Return the datetime value to use as Schedule Date (``date_planned``) for the
           Purchase Order Lines created to satisfy the given procurement.
           :param browse_record procurement: the procurement for which a PO will be created.
           :rtype: datetime
           :return: the desired Schedule Date for the PO lines
        """
        procurement_date_planned = datetime.strptime(procurement.date_planned, DEFAULT_SERVER_DATETIME_FORMAT)
        schedule_date = (procurement_date_planned - relativedelta(days=procurement.company_id.po_lead))
        return schedule_date
    @api.v8
    def _get_purchase_order_date(self, schedule_date):