I would like to have an indication (e.g. a tick mark) in the “view quotes” window (and in the quotes pane on the dashboard), that shows for which (approved) quotes an invoice has been sent. This would show me very quickly which projects haven’t been invoiced yet.
My workflow is that I first send a quote to the client, mark it as approved (if that’s indeed the case ;-)) and then, after the work is done I use the ‘quote to invoice’ option to create the corresponding invoice. For people who don’t use the ‘quote to invoice’ option it may be handy to add a ‘corresponding quote’ drop down list in the invoice creation window.