Consignment should not always be invoiced on assignment
Currently the consignment module creates invoice lines as soon as the move is assigned. I'm not sure if this behaviour is the desired one for most companies, but at least I think that some of them will want the invoice line to be created only when the move is moved to done state.
From my point of view, a good option would be that the default behaviour of stock_consignment module is to only create invoice lines when the move goes to done state and a new module can be created if somebody needs the invoice line to be created on assign.
Yet we could also provide a means for make this configurable in stock_consigment module.