Relatively few law departments track their lawyers’ time and charge it back to clients, like a law firm (See my post of Jan. 13, 2006 about disadvantages and April 27, 2005 about Kodak’s practices.) Those departments that do typically use the same billing rate for all their lawyers.
That is a better practice than charging different rates, because you don’t want clients lobbying for one lawyer as against another on cost. To challenge a lawyer’s billing rate would also be primarily a challenge of that lawyer’s compensation, the major determinate of the rate.
What does make sense, however, is to charge time for paralegals and to set the paralegal internal billing rate – a single rate, by the way – at a lower amount than the single lawyer rate.