I prefer being aggressive with Karthus in general. I'll be aggressive at first and see how my opponent react and play. If it goes well, I'll keep it up but if not, I'll fall back and last hit till 6 where I let my teammates know I can assists killing their lane.
You would be surprise sometime how someone playing a hard counter to Karth reacts when they face a aggressive Karth when they always faced him being passive. I got a few kills that way before they actually understood and I had to fall back to a more passive play.