Turn on "To be emailed" option for invoices only (not sales orders)

At present when the "Automatically check To be emailed" preference is selected it will turn this on for Sales Orders and Invoices. It would be useful to be able to select the preference just for invoices as we do not want to automatically send sales orders to customers but want to always send invoices. Staff forget to uncheck the to be emailed on SO and then these get sent in error.