From Boeing 777 FCTM:
Taxi-one engine:
"Because of additional operational procedural requirements and crew workload, taxiing out for flight with an engine shut down is not recommended. High bypass engines require warm up prior applying takeoff thrust and cool down prior to shutting down. If the engine has been shut down for several hours, it is desirable to operate as low a thrust setting as practicable for several minutes prior to takeoff.
If taxiing in after landing with an engine shut down, the crew must be aware of system requirements, ie, hydraulics, brakes, electrical. A minimum radius turn should be made in a direction that puts the operating engine at the outside of the turn.
In operational environments such as, uphill slope, soft asphalt, high gross weights, congested ramp areas, and wet/slippery ramps and taxiways, taxi with both engines operating"
My personal oppinion on this is that the workload is too high for the amount of fuel saved after a 12hr leg. It is hard to concentrate for normal duties and even more so with one engine out. It might be ok for 4 engine types, though. There is much more fuel to be saved by other means.