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.name = product_lang.display_name
        if product_lang.description_purchase:
            self.name += '\n' + product_lang.description_purchase
        fpos = self.order_id.fiscal_position_id
        if self.env.uid == SUPERUSER_ID:
            company_id = self.env.user.company_id.id
            self.taxes_id = fpos.map_tax(self.product_id.supplier_taxes_id.filtered(lambda r: r.company_id.id == company_id))
        else:
            self.taxes_id = fpos.map_tax(self.product_id.supplier_taxes_id)
        self._suggest_quantity()
        self._onchange_quantity()
        return result
    @api.onchange('product_qty', 'product_uom')
    def _onchange_quantity(self):
        if not self.product_id:
            return
        seller = self.product_id._select_seller(
            self.product_id,
            partner_id=self.partner_id,
            quantity=self.product_qty,
            date=self.order_id.date_order and self.order_id.date_order[:10],
            uom_id=self.product_uom)
        if seller or not self.date_planned:
            self.date_planned = self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        if not seller:
            return
        price_unit = self.env['account.tax']._fix_tax_included_price(seller.price, self.product_id.supplier_taxes_id, self.taxes_id) if seller else 0.0
        if price_unit and seller and self.order_id.currency_id and seller.currency_id != self.order_id.currency_id:
            price_unit = seller.currency_id.compute(price_unit, self.order_id.currency_id)
        if seller and self.product_uom and seller.product_uom != self.product_uom: