It's definitely more efficient to place the final cross edge incorrectly, but then again, Yau wasn't made for efficiency, it was made for speed and easy lookahead. With that in mind, I raise my move count a little bit, and place my edge in the correct place, in order to get easier thought posses, and thus easier lookahead, and higher TPS. My point: if you care about TPS, but the third cross edge in the right place, if you care about efficiency, put it in the wrong place.
As for Uw or Uw', I would say that it's best to be able to do both, as it lets you take advantage of free edges. anyway, its just preference, and so I do Uw.
Hope this helps.