Generally agree with Luke that there is no practical reason for continuing to account for basis/earnings after an employee reaches age 59 1/2 and satisfies the 5 year holding period. (I'm also assuming there's no state tax reason for doing so.) But IRS guidance may technically require it.
For example, the separate accounting regs, at (f)(3), state:
(3) Separate accounting required. Under the separate accounting requirement of this paragraph (f)(3), contributions and withdrawals of designated Roth contributions must be credited and debited to a designated Roth account maintained for the employee and the plan must maintain a record of the employee's investment in the contract (that is, designated Roth contributions that have not been distributed) with respect to the employee's designated Roth account. In addition, gains, losses, and other credits or charges must be separately allocated on a reasonable and consistent basis to the designated Roth account and other accounts under the plan. However, forfeitures may not be allocated to the designated Roth account and no contributions other than designated Roth contributions and rollover contributions described in section 402A(c)(3)(B) may be allocated to such account. The separate accounting requirement applies at the time the designated Roth contribution is contributed to the plan and must continue to apply until the designated Roth account is completely distributed. A-13 of § 1.402A-1 for additional requirements for separate accounting.
And the regs also contain this Q&A:
Q-7. After a qualified distribution from a designated Roth account has been made, how is the remaining investment in the contract of the designated Roth account determined under section 72?
A-7. (a) The portion of any qualified distribution that is treated as a recovery of investment in the contract is determined in the same manner as if the distribution were not a qualified distribution. (See A-3 of this section) Thus, the remaining investment in the contract in a designated Roth account after a qualified distribution is determined in the same manner after a qualified distribution as it would be determined if the distribution were not a qualified distribution.
However, the example the IRS provides deals with a qualified distribution due to disability, and notes that "This determination of the remaining investment in the contract will be needed if C subsequently is no longer disabled and takes a nonqualified distribution from the designated Roth account."