Taxes are considered to be "pay as you go". You do not have to withhold the money but the IRS does not want you to pay the following year either. You can make an estimated payment, but it is much easier to withhold the money.
If you didn't pay enough tax throughout
the year, either through withholding or by making estimated tax payments, you
may have to pay a penalty for underpayment of estimated tax. Generally,
most taxpayers will avoid this penalty if they either owe less than $1,000 in
tax after subtracting their withholding and estimated tax payments, or if they
paid at least 90% of the tax for the current year or 100% of the tax shown on
the return for the prior year, whichever is smaller.
