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()
        schedule_date = self._get_purchase_schedule_date()
        purchase_date = self._get_purchase_order_date(schedule_date)
        fpos = self.env['account.fiscal.position'].with_context(company_id=self.company_id.id).get_fiscal_position(partner.id)
        gpo = self.rule_id.group_propagation_option
        group = (gpo == 'fixed' and self.rule_id.group_id.id) or \
                (gpo == 'propagate' and self.group_id.id) or False
        return {
            'partner_id': partner.id,
            'picking_type_id': self.rule_id.picking_type_id.id,
            'company_id': self.company_id.id,
            'currency_id': partner.property_purchase_currency_id.id or self.env.user.company_id.currency_id.id,
            'dest_address_id': self.partner_dest_id.id,
            'origin': self.origin,
            'payment_term_id': partner.property_supplier_payment_term_id.id,
            'date_order': purchase_date.strftime(DEFAULT_SERVER_DATETIME_FORMAT),
            'fiscal_position_id': fpos,
            'group_id': group
        }
    @api.multi
    def make_po(self):
        cache = {}
        res = []
        for procurement in self:
            suppliers = procurement.product_id.seller_ids.filtered(lambda r: not r.product_id or r.product_id == procurement.product_id)