The answer depends on the structure of the retirement vehicle(s). Many are designed to defer the taxable event (typically tax on earnings) until retirement or disbursement even later, while others are designed to have immediate taxable impact with no taxable effect on earnings. Roth IRAs are in this second category. Put a different way, they can be either pay later or pay now.

In my experience, most are in the pay-later category, and they have penalties if funds residing there are accessed prior to a minimum age (usually late 50s or early 60s). If that's where your funds are going, then your tax burden for the year would be reduced, resulting in a larger refund or lower tax payment.

Contributions to a traditional 401(k) serve to decrease your taxable income (shown on Form W-2).
Contributions to a Roth 401 will not decrease your taxable income.
Contributing to a traditional IRA might decrease your taxable income, it depends on how much income there is on the tax return.
Contributing to a Roth IRA does not change your taxable income.
If you are self-employed, then contributing to a SEP, SIMPLE or Keogh should decrease your total tax.

For some lower-income taxpayers, contributing to a retirement plan might make you eligible for a small "Saver's Credit."

Thanks