"For whom do you cook?"
Translation:Σε ποιον μαγειρεύεις;
It sounds odd to me.
Putting a preposition at the end of the sentence is pretty colloquial, and frowned on by more conservative speakers.
Using "whom" rather than "who" is a little formal.
The combination of those two seems odd to me.
"Who are you cooking for?" (colloquial) or "For whom are you cooking?" (formal) would be better, I think.
It sounds fine to my ear. The two issues (correctly using whom and not placing prepositions at the end) are completely unrelated.
That said, if you choose to begin a sentence with a preposition, your hand is forced: you must use "whom" after a preposition (or else you will sound like a high school dropout): A woman to whom I am married, a man for whom I can vouch etc... If you choose to place the preposition a the end (which is looked down upon only by the most anal Latin-inspired purists), you still have an option of properly using the objective case for an object.
P.S. You don't have to take my word for it: just google "whom she was afraid of". Here is one link: http://www.newyorker.com/magazine/2006/06/05/dimension