I would recommend against any even recognition of the word whom. It's only ever used by radical English teachers, who ONLY speak English, and don't ACTUALLY have any idea what a direct object really is. Кого is great. Russian has actual use for these things, but that ship has sailed for english a long time ago (sailed, crashed, and sunk). Its 100% word order now. Its at the point now where, if you DO say whom, it will often stop the conversation dead in its tracks and make the person give you a disappointed/confused look. "My boss, who I hate..." is the standard now. All the way up (on the formality scale) to newspapers have forgotten "whom". If you're writing legal documents, it'll be okay. If you're in a highly professional (perhaps more language based, such as legal) environment, using it can then seem more "educated", than know-it-all/nerdy/attention-seeking. When it comes to "whom" as in the object of a preposition: "Who are you looking at?" is standard, "At whom are you looking?" is pretty much an unheard sentence entirely.