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
self.ensure_one()
        line = self[0]
        order = line.order_id
        price_unit = line.price_unit
        if line.taxes_id:
            price_unit = line.taxes_id.with_context(round=False).compute_all(price_unit, currency=line.order_id.currency_id, quantity=1.0)['total_excluded']
        if line.product_uom.id != line.product_id.uom_id.id:
            price_unit *= line.product_uom.factor / line.product_id.uom_id.factor
        if order.currency_id != order.company_id.currency_id:
            price_unit = order.currency_id.compute(price_unit, order.company_id.currency_id, round=False)
        return price_unit
    @api.multi
    def _create_stock_moves(self, picking):