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):

addons/purchase/purchase.py
uom_id=self.product_id.uom_po_id)
        taxes = self.product_id.supplier_taxes_id
        fpos = po.fiscal_position_id
        taxes_id = fpos.map_tax(taxes) if fpos else taxes
        if taxes_id:
            taxes_id = taxes_id.filtered(lambda x: x.company_id.id == self.company_id.id)
        price_unit = self.env['account.tax']._fix_tax_included_price(seller.price, self.product_id.supplier_taxes_id, taxes_id) if seller else 0.0
        if price_unit and seller and po.currency_id and seller.currency_id != po.currency_id:
            price_unit = seller.currency_id.compute(price_unit, po.currency_id)
        product_lang = self.product_id.with_context({
            'lang': supplier.name.lang,
            'partner_id': supplier.name.id,
        })
        name = product_lang.display_name
        if product_lang.description_purchase:
            name += '\n' + product_lang.description_purchase
        date_planned = self.env['purchase.order.line']._get_date_planned(seller, po=po).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        return {
            'name': name,
            'product_qty': procurement_uom_po_qty,
            'product_id': self.product_id.id,
            'product_uom': self.product_id.uom_po_id.id,
            'price_unit': price_unit,
            'date_planned': date_planned,
            'taxes_id': [(6, 0, taxes_id.ids)],
            'procurement_ids': [(4, self.id)],
            'order_id': po.id,
        }
    @api.multi
    def _prepare_purchase_order(self, partner):