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):

READONLY_STATES = {
        'purchase': [('readonly', True)],
        'done': [('readonly', True)],
        'cancel': [('readonly', True)],
    }
    name = fields.Char('Order Reference', required=True, index=True, copy=False, default='New')
    origin = fields.Char('Source Document', copy=False,\
        help="Reference of the document that generated this purchase order "
             "request (e.g. a sale order or an internal procurement request)")
    partner_ref = fields.Char('Vendor Reference', copy=False,\
        help="Reference of the sales order or bid sent by the vendor. "
             "It's used to do the matching when you receive the "
             "products as this reference is usually written on the "
             "delivery order sent by your vendor.")
    date_order = fields.Datetime('Order Date', required=True, states=READONLY_STATES, index=True, copy=False, default=fields.Datetime.now,\
        help="Depicts the date where the Quotation should be validated and converted into a purchase order.")
    date_approve = fields.Date('Approval Date', readonly=1, index=True, copy=False)
    partner_id = fields.Many2one('res.partner', string='Vendor', required=True, states=READONLY_STATES, change_default=True, track_visibility='always')
    dest_address_id = fields.Many2one('res.partner', string='Drop Ship Address', states=READONLY_STATES,\
        help="Put an address if you want to deliver directly from the vendor to the customer. "\
             "Otherwise, keep empty to deliver to your own company.")
    currency_id = fields.Many2one('res.currency', 'Currency', required=True, states=READONLY_STATES,\
        default=lambda self: self.env.user.company_id.currency_id.id)
    state = fields.Selection([
        ('draft', 'Draft PO'),
        ('sent', 'RFQ Sent'),
        ('to approve', 'To Approve'),
        ('purchase', 'Purchase Order'),
        ('done', 'Done'),
        ('cancel', 'Cancelled')