The IRS essentially requires that all Forms 1099-R be accounted for on the individual's tax return. Although it should not be possible, direct rollovers (codes B and G together for a direct rollover from a Roth 401(k)) sometimes fail because they are inappropriately diverted to non-retirement accounts, so the reporting the nontaxable rollover on the tax return is your attestation to the fact that the rollover was actually completed. Of course a rollover to an IRA will also be reported by the IRA custodian on Form 5498, but that does not release you from the obligation to have the rollover properly shown on your tax return. The same codes can indicate a rollover to a Roth account in another qualified retirement plan rather than to a Roth IRA, and under those circumstances there would be no Form 5498 to indicate that the rollover had been completed. Also rollovers done late in the year might not be deposited into the receiving account until early the following year, in which case the Form 5498 for the deposit into the IRA would not be generated until more than a year after the year of the tax return on which the distribution is required to be reported.
To summarize, you are required to report the nontaxable rollover on your tax return to give the IRS the necessary confidence that the rollover was actually completed.
