There are two separate areas of your tax return where the year-end value of your traditional IRAs might be needed. First, if you've made an excess contribution to your traditional IRAs, the year-end value is needed to determine the penalty since the penalty determined on Form 5329 is 6% of the lesser of the excess or they year-end value. Second, the year-end value is needed if you made any taxable distributions from your traditional IRAs and you have basis in nondeductible traditional IRA contributions, where the taxable amount needs to be determined on Form 8606. In this second case, TurboTax might ask for the year end value before actually determining if Form 8606 is needed. TurboTax asks for the year-end value separately for these cases, so it might ask twice and, if so, would need to be entered twice (or maybe even three times if you also have Roth IRAs that might have excess contributions and TurboTax needs to know the year-end value of your Roth IRAs).
Failing to provide the year-end value when TurboTax asks for it could result in an incorrectly prepared Form 5329 or Form 8606. Also, jumping around in the TurboTax entry process could result in skipping the question when the value is actually needed, and TurboTax generally fails to catch and alert you about such an omission.
The IRA custodian is required to include the year-end value on your IRA year-end statement, so that's where you would typically look for the year-end value since the Form 5498 which reports the year-end value to the IRS is often not issued in time to help in preparing your tax return.
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