The new 'default tax date' that uses either the Posting Date or Document Date is very good and already being used by a few of our clients. It could be improved if the batch post sales document routine (which allows us to change the posting date as one of the options) also honored this functionality. A good way to do it would be to add an option 'Update Tax Date' which would change the tax date if the posting date is being over-ridden and that drives the default tax date.