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
args = args or []
        domain = []
        if name:
            domain = ['|', ('name', operator, name), ('partner_ref', operator, name)]
        pos = self.search(domain + args, limit=limit)
        return pos.name_get()
    @api.multi
    @api.depends('name', 'partner_ref')
    def name_get(self):
        result = []
        for po in self:
            name = po.name
            if po.partner_ref:
                name += ' ('+po.partner_ref+')'
            result.append((po.id, name))
        return result
    @api.model
    def create(self, vals):