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
def action_set_date_planned(self):
        
        date_planned = self.env.context.get('date_planned')
        if not date_planned:
            return
        for order in self:
            for line in order.order_line:
                line.update({'date_planned': date_planned})
class PurchaseOrderLine(models.Model):
    _name = 'purchase.order.line'
    _description = 'Purchase Order Line'
    @api.depends('product_qty', 'price_unit', 'taxes_id')
    def _compute_amount(self):
        for line in self:
            taxes = line.taxes_id.compute_all(line.price_unit, line.order_id.currency_id, line.product_qty, product=line.product_id, partner=line.order_id.partner_id)
            line.update({
                'price_tax': taxes['total_included'] - taxes['total_excluded'],
                'price_total': taxes['total_included'],
                'price_subtotal': taxes['total_excluded'],
            })
    @api.depends('invoice_lines.invoice_id.state')
    def _compute_qty_invoiced(self):
        for line in self:
            qty = 0.0
            for inv_line in line.invoice_lines:
                if inv_line.invoice_id.state not in ['cancel']:
                    qty += inv_line.uom_id._compute_qty_obj(inv_line.uom_id, inv_line.quantity, line.product_uom)
            line.qty_invoiced = qty
    @api.depends('order_id.state', 'move_ids.state')
    def _compute_qty_received(self):