You will pay less tax, but it won't necessarily lower your "tax bill" when you file your tax return.
If you contribute to a retirement account through your employer, your employer will reduce your taxable income by that amount (Box 1 of the W-2), but they will ALSO withhold less income tax.
If you want to avoid a large bill when filing your tax return, you should have more tax taken out of your paycheck (give your employer a new Form W-4) or pay Estimated Tax payments.
