Thanks to the federal Making Work Pay Credit, most Americans have a few extra dollars in their take home pay.

While enjoying that additional income, IRS officials are encouraging workers to closely monitor their tax withholding.

The federal tax credit has lowered tax withholding rates for around 120 million households, IRS officials report. The credit, which was made possible by adjusting existing tax tables, may also create a few surprises for some uninformed tax filers next spring.

"We want to make sure that individuals are having enough taxes withheld, but we also want to make sure that too many taxes aren’t being taking out of their pay," said Mark Green, IRS spokesperson. "The goal here is for people to not wait until the end of the tax season to check things out."

With the year half over, IRS officials says now is the perfect time to assess tax withholding because there is still time to make effective adjustments. To make things easier, the IRS has posted a withholding calculator on its Web site.

To use the calculator, taxpayers need their most recent pay stub and income tax return information. All information provided is anonymous and won’t be used by the IRS to try and identify the person.

Although all taxpayers should be attentive, married couples with two incomes should especially keep an eye out, Green says.

"For instance, if a working couple has one child and the wife claims the child, then she is eligible for two allowances. If the husband also claims two allowances then they may have a very small refund or owe a tax liability," Green said.

"If taxpayers usually receive a very large refund, like thousands of dollars, they may also want to take a look at their withholding because too much tax is probably being withheld. If they adjust the withholding, it would in a sense be like giving yourself a raise.

"On the other hand, if you (owe taxes) every year, you may also want to make an adjustment to your withholding to off set (the taxes that you owe)."

Pensioners, workers claimed as dependents by other taxpayers and some Social Security recipients who work should pay close attention to their withholding amount.

Even those individuals who receive unemployment benefits should keep an eye on withholding allowances — although the first $2,400 of benefits received in 2009 are federal tax free, after that the income is subject to taxation.

"People who expect to receive more than that should consider having tax withheld from their benefit payments in excess of $2,400. Use Form W-4V, Voluntary Withholding Request, or the equivalent form provided by the payer to request withholding to begin or end," say IRS officials.