Good Question! Documents which have outlived their practical and legal usefulness should be properly destroyed. Adherence to a predetermined retention schedule is a sound business practice which reduces risks of litigation and audit. We have a nice reference on document retention here.

Document Retention Schedule

One Year:

• Correspondence of unimportant nature with customers or vendors
• Purchase orders (except purchasing department copies)
• Receiving sheets
• Requisitions
• Stenographers' notebooks
• Stockroom withdrawal forms 

Seven Years:

• Accident reports
• Accounts receivable ledgers and related trial balances
• Bank reconciliations
• Bank statements
• Canceled checks (see exception under permanent records)
• Contracts and leases expired
• Duplicate deposit slips
• Expense analyses and expense distribution schedules
• Garnishments
• Inventory summaries of product, materials and supplies
• Invoices to customers and from vendors Journal vouchers
• Notes receivable ledgers and related trial balances
• Payroll records and summaries, including payments to pensioners
• Personnel files (terminated)
• Purchase orders (purchasing department copies)
• Sales records Scrap salvage records -
• Inventories, sales, etc. Subsidiary ledgers to the general ledger and related trial balances
• Time books
• Voucher register and related trial balances