If you're tanking a raid I'll just assume you buff yourself with food. If not, then you're doing it wrong.
For a five man heroic, though, the answer isn't as clear cut. For a tank in 350+ gear it's simply not needed, especially not in the old heroics.
Now, the troll instances present an interesting dilemma. Threat can sometimes be an issue, because for us casuals the mobs still hit hard enough for us to wear a survival set.
So, how to get the good without paying too much for it?
The Tol Barad dailies is a very sweet help here. Killing crocs give us what we need to produce stamina/expertise food. Unless you're wiping a lot in five mans even the modest amount of food available from doing those dailies will give us a pretty nice chunk of extra threat.
Oh, and do kill the guy on the bridge in Zul'Farrak. Someone in the party will get the ingredients for a full stack of food. Maybe not you, but the mob dies in just about no time. After all, you do take the time to do the lottery for one Maelstrom Crystal.