The IRS insists records of employment taxes be kept for at least four years, and files should include:
Amounts and dates of all wage, annuity and pension
payments.
Names, addresses, Social Security numbers and occupations
of employees, as well as their dates of employment.
Employee copies of Form W-2 returned as undeliverable.
The fair market value of in-kind wages paid.
Copies of employees’ income tax withholding allowance
certificates.
Dates and amounts of tax deposits.
Copies of returns filed.
Records of allocated tips and fringe benefits provided,
including substantiation.
Source: www.irs.gov.