User blog comment:Krufix/Last Whisper - When to get?/@comment-4022742-20121106103625

Personally I get a Last whisper when 3 or more opponents have more then 100 armor. Because otherwise you are statistically better off building a Black Cleaver. Getting a black cleaver as your first damage item after zerkers, zeal and vamp scepter is valid since it increases your AD, ASPD and armor penetration early on getting you kills.

However if you have to do this for a Last whisper I'd say you are better off building Zerkers -> zeal -> vamp scepter -> IE -> Last whisper -> PD -> BT... Since this will increase your attack speed, damage and sustain before you have to worry about omitting their armor.

So second rule for me would be building at least 1 major damage item, either IE or BT is fine.