In the 747, Approach Idle is active until 5 seconds after touchdown, and when thrust reversers are selected.
Approach idle for five seconds after touchdown is standard design practice for all Boeing aircraft (at least the Puget Sound variety - don't know about the former McD Long Beach types). Several years ago the question came up as to the original source of the requirement for a five second delay after the air/ground transition. Best we could come up with was "we've always done it that way"
On the 747-400, approach idle during T/R selection is only the case on the GE CF6-80C2 installation - not so for Pratt and Rolls. This is because the CF6-80C2 uses a pneumatic reverser and approach idle is needed to insure adequate bleed pressure for T/R operation (Pratt and Rolls on the -400 have hydraulic reversers).
The 747-8 has hydraulic reversers and doesn't not select approach idle during T/R selection.