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 vals.get('name', 'New') == 'New':
            vals['name'] = self.env['ir.sequence'].next_by_code('purchase.order') or '/'
        return super(PurchaseOrder, self).create(vals)
    @api.multi
    def unlink(self):
        for order in self:
            if not order.state == 'cancel':
                raise UserError(_('In order to delete a purchase order, you must cancel it first.'))
        return super(PurchaseOrder, self).unlink()
    @api.multi
    def copy(self, default=None):
        new_po = super(PurchaseOrder, self).copy(default=default)
        for line in new_po.order_line:
            seller = line.product_id._select_seller(
                line.product_id, partner_id=line.partner_id, quantity=line.product_qty,
                date=line.order_id.date_order and line.order_id.date_order[:10], uom_id=line.product_uom)
            line.date_planned = line._get_date_planned(seller)
        return new_po
    @api.multi
    def _track_subtype(self, init_values):
        self.ensure_one()
        if 'state' in init_values and self.state == 'purchase':
            return 'purchase.mt_rfq_approved'
        elif 'state' in init_values and self.state == 'to approve':
            return 'purchase.mt_rfq_confirmed'
        elif 'state' in init_values and self.state == 'done':
            return 'purchase.mt_rfq_done'
        return super(PurchaseOrder, self)._track_subtype(init_values)
    @api.onchange('partner_id', 'company_id')
    def onchange_partner_id(self):