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
if not self.partner_id:
            self.fiscal_position_id = False
            self.payment_term_id = False
            self.currency_id = False
        else:
            self.fiscal_position_id = self.env['account.fiscal.position'].with_context(company_id=self.company_id.id).get_fiscal_position(self.partner_id.id)
            self.payment_term_id = self.partner_id.property_supplier_payment_term_id.id
            self.currency_id = self.partner_id.property_purchase_currency_id.id or self.env.user.company_id.currency_id.id
        return {}
    @api.onchange('picking_type_id')
    def _onchange_picking_type_id(self):
        if self.picking_type_id.default_location_dest_id.usage != 'customer':
            self.dest_address_id = False
    @api.multi
    def action_rfq_send(self):
        '''
        This function opens a window to compose an email, with the edi purchase template message loaded by default
        '''
        self.ensure_one()
        ir_model_data = self.env['ir.model.data']
        try:
            if self.env.context.get('send_rfq', False):