I think most students of the great USAF tanker contest would accept that the key weakness of the Boeing offering against the Airbus proposal is that the 767 doesn't have winglets. The USA has a certain image to keep up and you simply can't go global policing these days without the right accessories. For example, if you're special forces then you don't want to be seen without your Oakleys. And, whereas until now every tanker jock ideally wanted four Conways and a screw-you T-tail, the new generation want winglets like the airline guys. Or even like the trash-haulers for God's sake!
Fortunately for them, the Air Force Studies Board thinks there all sorts of operational arguments backing up the more image-related ones for trying winglets on the KC-135 and KC-10.
Turns out it's done quite a bit of work on the question. And it reckons that most transports, including sadly the C-5, are not wingletting candidates, since the Air Force would have to pay for the development. But the KC-135 could see big benefits, and there is even a pretty good argument for the KC-10.
In fact in the mid-70s Boeing calculated a performance gain of 7.5% if the KC-135 was wingletted. But that was with JT3D engines and the actual gain may be less now. What I didn't know was that NASA actually flew such a beast - and the numbers came out about right.
And I certainly didn't even guess that they also flew a wingletted DC-10.
Here it is, a Continental Airlines DC-10-10.
That got a 2-3% gain, and the MD-11 was much the same.
However, fashion is a fickle thing, and since the current Boeing KC-X proposal seems to include the 767-400ER wings then the Air Force could be sporting those rather snappy raked tips instead.
I suppose it's all a question of future proofing.