If you can get hold of a copy of 'Handling the Big Jets' by D. P. Davies, there is a very good discussion on just these points. On pages 156 - 159 he discusses emergency descents in general and the relative advantages of gear up vv gear down, and, then on pages 283 - 286, he discusses the 747 Classic.
There are a number of considerations to be taken into account. First, if you use the gear down method, you will probably need to stay at altitude for around 20 to 30 seconds while you decelerate to the gear lowering limiting speed whereas, as others have mentioned on this thread, it is probably better to start going down immediately. Second, you will need to consider the possibility of reduced structural integrity therefore descending at a slower speed may be a good idea. Third, as has been mentioned by mustafagander, if you have any doubts about the hydraulic system and the ability to raise the gear again, and you are way out over the ocean, you wouldn't want to have the range compromised by the drag of the gear remaining down.
All these are command decisions that can only be made at the time!