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):

"""Return the datetime value to use as Schedule Date (``date_planned``) for
           PO Lines that correspond to the given product.seller_ids,
           when ordered at `date_order_str`.
           :param browse_record | False product: product.product, used to
               determine delivery delay thanks to the selected seller field (if False, default delay = 0)
           :param browse_record | False po: purchase.order, necessary only if
               the PO line is not yet attached to a PO.
           :rtype: datetime
           :return: desired Schedule Date for the PO line
        """
        date_order = po.date_order if po else self.order_id.date_order
        if date_order:
            return datetime.strptime(date_order, DEFAULT_SERVER_DATETIME_FORMAT) + relativedelta(days=seller.delay if seller else 0)
        else:
            return datetime.today() + relativedelta(days=seller.delay if seller else 0)
    @api.onchange('product_id')
    def onchange_product_id(self):
        result = {}
        if not self.product_id:
            return result
        
        self.date_planned = datetime.today().strftime(DEFAULT_SERVER_DATETIME_FORMAT)
        self.price_unit = self.product_qty = 0.0
        self.product_uom = self.product_id.uom_po_id or self.product_id.uom_id
        result['domain'] = {'product_uom': [('category_id', '=', self.product_id.uom_id.category_id.id)]}
        product_lang = self.product_id.with_context({
            'lang': self.partner_id.lang,
            'partner_id': self.partner_id.id,
        })