Roth contributions and Roth conversions are reported to both you and the IRS on Forms 5498 prepared by the Roth IRA custodian. If the IRS is so inclined, the IRS could construct your Roth IRA basis history from these and from Forms 1099-R reporting distributions from your Roth IRAs to determine if any reporting you might make on Form 8606 regarding the taxability of a nonqualified Roth IRA distribution is accurate. Still, it's your responsibility to track your Roth IRA basis so that you can accurately prepare Part III of Form 8606 if it's ever needed.
