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/sale/sale.py
validity_date = fields.Date(string='Expiration Date', readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]})
    create_date = fields.Datetime(string='Creation Date', readonly=True, index=True, help="Date on which sales order is created.")
    user_id = fields.Many2one('res.users', string='Salesperson', index=True, track_visibility='onchange', default=lambda self: self.env.user)
    partner_id = fields.Many2one('res.partner', string='Customer', readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, required=True, change_default=True, index=True, track_visibility='always')
    partner_invoice_id = fields.Many2one('res.partner', string='Invoice Address', readonly=True, required=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, help="Invoice address for current sales order.")
    partner_shipping_id = fields.Many2one('res.partner', string='Delivery Address', readonly=True, required=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, help="Delivery address for current sales order.")
    pricelist_id = fields.Many2one('product.pricelist', string='Pricelist', required=True, readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, help="Pricelist for current sales order.")
    currency_id = fields.Many2one("res.currency", related='pricelist_id.currency_id', string="Currency", readonly=True, required=True)
    project_id = fields.Many2one('account.analytic.account', 'Analytic Account', readonly=True, states={'draft': [('readonly', False)], 'sent': [('readonly', False)]}, help="The analytic account related to a sales order.", copy=False, domain=[('account_type', '=', 'normal')])
    order_line = fields.One2many('sale.order.line', 'order_id', string='Order Lines', states={'cancel': [('readonly', True)], 'done': [('readonly', True)]}, copy=True)
    invoice_count = fields.Integer(string='# of Invoices', compute='_get_invoiced', readonly=True)
    invoice_ids = fields.Many2many("account.invoice", string='Invoices', compute="_get_invoiced", readonly=True, copy=False)
    invoice_status = fields.Selection([
        ('upselling', 'Upselling Opportunity'),
        ('invoiced', 'Fully Invoiced'),
        ('to invoice', 'To Invoice'),
        ('no', 'Nothing to Invoice')
        ], string='Invoice Status', compute='_get_invoiced', store=True, readonly=True, default='no')
    note = fields.Text('Terms and conditions', default=_default_note)
    amount_untaxed = fields.Monetary(string='Untaxed Amount', store=True, readonly=True, compute='_amount_all', track_visibility='always')
    amount_tax = fields.Monetary(string='Taxes', store=True, readonly=True, compute='_amount_all', track_visibility='always')
    amount_total = fields.Monetary(string='Total', store=True, readonly=True, compute='_amount_all', track_visibility='always')
    payment_term_id = fields.Many2one('account.payment.term', string='Payment Term', oldname='payment_term')
    fiscal_position_id = fields.Many2one('account.fiscal.position', oldname='fiscal_position', string='Fiscal Position')
    company_id = fields.Many2one('res.company', 'Company', default=lambda self: self.env['res.company']._company_default_get('sale.order'))
    team_id = fields.Many2one('crm.team', 'Sales Team', change_default=True, default=_get_default_team, oldname='section_id')
    procurement_group_id = fields.Many2one('procurement.group', 'Procurement Group', copy=False)
    product_id = fields.Many2one('product.product', related='order_line.product_id', string='Product')
    @api.model
    def _get_customer_lead(self, product_tmpl_id):