Check your federal withholding, especially if you've married, had children, bought a home, earned investment income, or made capital gains profit in 2012. Conventional wisdom says withhold just enough tax to meet your tax liability. Although withholding too much means a refund, you're also giving Uncle Sam an interest-free loan. Withhold too little and you'll owe taxes plus possible penalties and interest. You can change withholding anytime of year by submitting a revised Form W-4 to your employer.