first off, ICC bosses hit less hard than they would of if they hadn't put in chill of the throne, they still will hit harder than ToC, though normal modes are pretty straightforward at the moment and don't really hurt that much.
Secondly, gemming will probably depend on fights. I actually plan on having 2 sets of gear, 1 for EHP and one for Avoidance. (someone wanna chip in to buy me 4 24 slot bags? lol). For the most part right now, keep gemming stam, the further bosses in may be more challenging, but for the time being it doesn't matter much. Once heroics come out, some fights like saurfang and gunship may require tanks to have a raw avoidance set because of fight mechanics.
Gem based on the fight is basically what I'm saying, and have different pieces of gear that you can throw on for different scenarios.