If you add an invoice line or change an amount, the total are not updated. There is a button "update" next to tax to recalculate the tax and the total. You can save the invoice without recalculating, but you cannot validate the invoice - at least in my instance (v8).

I am not sure whether there is a setting to do this automatcally on save. But I think you could do this by modifying the code. addons/account/account_invoice.py. There is a method called action_move_create. I think if you add a call at the beginning of that function to self.button_compute(self,cr,uid,ids,content), that should do the trick. But not tested and I have no experience in python.