The 777 wing is already very efficient, so any efficiency gains achieved by adding winglets would likely be offset by the additional weight and expense of the winglets themselves. Note that the 777 without winglets has the same range as, and a higher cruising speed than, its winglet-equipped rival, the A340-300. Different designs work well for different planes.
However, for the "Longer Range" 777s, Boeing evidently decided that the benefits produced by raked wingtips would offset the increased weight (as you may know, raked wingtips have a similar aerodynamic effect to winglets). So, in the design of any plane, it's a judgment call based on the balance of costs and benefits.
FYI, you might get somewhat better answers to questions like this on the Tech/Ops board, in part because there tend to be fewer Airbus v. Boeing arguments and less sarcasm in general over there, but I hope this helps a little bit.
And finally, don't be put off by the "already discussed before" police. The search function does not always work very well, and if you're new to the board it's inevitable that you'll occasionally ask something that has been discussed at some point in the past. There are people here who get their kicks out of pointing that out...just ignore them.
Common sense is not so common. -Voltaire