Regarding the Sennheiser plugs, very good! Also, when I usually build such a cable with those connectors, I use a small toothpick and apply a small drop of paint on the shrinkwrap on the "inside" side. This allows for one to easily determine which direction the plugs should be inserted for the L and the R.
Dont they only go in one way? Isnt that why one pin in each is larger than the other?