Keep them if you anticipate having to reprint them, for a reasonable time frame. And only then, keep them in History instead of Current (if you are using OE) to help improve performance of dealing with live transactions. If you don't purge transactions from OE, processing and reports takes longer than it would with fewer transactions.

If you are keeping documents solely for sales reporting, you should investigate Adagio SalesAnalysis and store the documents there (instead of OE or Invoices) when you don't need to re-print anymore.