Roth conversions are unconditionally required to be reported on Form 8606 Part II. When you have basis in nondeductible traditional IRA contributions, Part I is also required to be completed where the pro rata calculation is done. As Opus 17 said, a pro rata portion of your basis in nondeductible traditional IRA contributions applies to the distribution whether you file a correct Form 8606 or not, so there is no reason not to prepare Form 8606 correctly so that the nontaxable portion is not taxed. TurboTax will ask the necessary questions to obtain the information needed to complete Form 8606.
