Roth and traditional IRA's are not reported on a W-2 form. Employer sponsored retirement plans, such as a 401k, would be reported on a W-2.
If you contributed to a retirement plan via payroll deductions, then you would not enter contributions in the Deductions & Credit section. Your W-2 entry is all that is required.
If you initiated a Roth or traditional IRA on your own, separate from your employer through a bank, mutual fund or brokerage, then you would enter your contributions under the Deductions & Credits section.