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
for order in self:
            invoices = self.env['account.invoice']
            for line in order.order_line:
                invoices |= line.invoice_lines.mapped('invoice_id')
            order.invoice_ids = invoices
            order.invoice_count = len(invoices)
    @api.model
    def _default_picking_type(self):
        type_obj = self.env['stock.picking.type']
        company_id = self.env.context.get('company_id') or self.env.user.company_id.id
        types = type_obj.search([('code', '=', 'incoming'), ('warehouse_id.company_id', '=', company_id)])
        if not types:
            types = type_obj.search([('code', '=', 'incoming'), ('warehouse_id', '=', False)])
        return types[:1]
    @api.depends('order_line.move_ids')
    def _compute_picking(self):
        for order in self:
            pickings = self.env['stock.picking']
            for line in order.order_line:
                
                
                moves = line.move_ids | line.move_ids.mapped('returned_move_ids')
                moves = moves.filtered(lambda r: r.state != 'cancel')
                pickings |= moves.mapped('picking_id')
            order.picking_ids = pickings
            order.picking_count = len(pickings)