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It depends really. If the enemy team is stupid (as in for some reason they are building no armor) then you can choose to build it after BT. Usually, though, if you play against competent people, if you're doing good they will build armor so you will need that LW to be able to hurt them. If the enemy team is going after you, it will most likely be the high armor champions chasing you, so being able to kill them quicker is better. BC is nice, its just that most tanks/tankydps will have 150-200ish armor easily, which is when LW starts being better then BC. This is just my preference though.
Also, in your IE build, you should have getting a vamp scepter, since its helps a lot with sustain and in 1v1's or 2v2's.