on winglets and raked tips
Winglets need to be designed-in from the start, as the optimum twist distribution along the wing span will be different with/without winglets. In general, you need more tip-loading for a wing-winglet configuration to work. Take a look at the 747 version with winglets. You can clearly see that they had to re-twist the outer wing section as well as adding winglets to get them to work.
With regards to raked tips they work in a similar fashion to winglets. It is, infact the shape of the trailing edge of the wing that matters, as viewed from behind (the shape in the Trefftz-plane if we want to get a bit technical). As the wing is pitched nose-up at cruise, the trailing edge line curves down at the tip a bit like a downwards-pointing winglet.
As the aim is to spread the vorticity over as long a length as possible in a non-planar fashion, it makes no difference if the winglet or trailing edge shape at the tip is pointing up or down.
Of course the natural world evolved this way before Boeing did....although that hasn't stopped them trying to claim a patent on it. They are also called 'lunate' or 'crescent moon' configurations and you see them in the wing planform shape of the european swift (if you stay in the air for a whole year without landing, you can be sure your wing planform will evolve to be optimum...that fact regarding swifts still takes my breath away). Also check out the tail fins of tuna, sword fish, dolphins and whales....maybe dolphins really are the most intelligent life form on the planet......bye and thanks for all the fish.
A cresent-moon planform on an aircraft will be very expensive to manufacture, so just adding the feature to the tip area makes more sense - the raked tip.
Raked-tips on aircraft also have one aerodynamic advantage over winglets - with winglets the main design problem is the interference you get in the wing/winglet junction. Profile drag and wave drag penalties can occur here. You don't get this penalty with a raked tip.
With larger aircraft, wing weight becomes more important, in which case it is better to have a span loading with a more inboard centre of pressure to reduce wing root bending moment and so wing weight. In this case, winglets dont work so well. This is why A380 has relatively small winglets/end-plates (relative to the wing span that is!).
Hope this helps.