I used to mark targets just to make tanking easier for me, but after getting rage of halone, I stopped marking after the first target, and now I don't mark any targets unless people ask me to. Why? Because I'm a lazy tank and it keeps me on my toes when I have slight competition going on with the DDs. Otherwise tanking would be as boring as playing my low level conjurer: "1, 2, 3... 1, 2, 3... 1, 2, 3..." Besides, it helps me learn how to keep multiple targets on me without relying too much on Flash.

But like I said, if people ask me to mark stuff, of course I will.