This would be a beneficial feature and would keep my internal documentation and business books consistent; and in my humble opinion, the perception would appear more professional (and detailed).  

I would like to be able to customize the invoice number (the second most important number on the receipt ;)) and then be able to methodically assign a unique number to each invoice I provide to that customer with a means to identify it.

Again the biggest benefit of using a numbering system, is that I can search for an invoice or locate a copy of the invoice in the documents within my own accounting books.