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):

self.ensure_one()
        seller_delay = int(self.product_id._select_seller(self.product_id).delay)
        return schedule_date - relativedelta(days=seller_delay)
    @api.v7
    def _get_purchase_order_date(self, cr, uid, procurement, company, schedule_date, context=None):
        """Return the datetime value to use as Order Date (``date_order``) for the
           Purchase Order created to satisfy the given procurement.
           :param browse_record procurement: the procurement for which a PO will be created.
           :param browse_report company: the company to which the new PO will belong to.
           :param datetime schedule_date: desired Scheduled Date for the Purchase Order lines.
           :rtype: datetime
           :return: the desired Order Date for the PO
        """
        seller_delay = int(procurement.product_id._select_seller(procurement.product_id).delay)
        return schedule_date - relativedelta(days=seller_delay)
    @api.multi
    def _prepare_purchase_order_line(self, po, supplier):
        self.ensure_one()
        procurement_uom_po_qty = self.env['product.uom']._compute_qty_obj(self.product_uom, self.product_qty, self.product_id.uom_po_id)
        seller = self.product_id._select_seller(
            self.product_id,
            partner_id=supplier.name,
            quantity=procurement_uom_po_qty,
            date=po.date_order and po.date_order[:10],