CTC, short for Combat Table Coverage, is how close a tank is to being unhittable. The goal is 102.4% to make sure that every hit is either a Block, Parry, Dodge or Miss. (dodge % + parry % + block chance % + 5 % miss base). For both classes of shield tank, Paladin and Warrior, the consensus is that for fights with mostly physical damage, reaching the CTC cap by stacking mastery is a higher priority than dodge/parry or stamina. Beyond the CTC cap, many tanks move their gemming towards stamina, rather than the other option of higher dodge/parry.[1]
For high magic damage fights (heroic Yor'sahj has more than 70% of the total damage on the tank(s) as shadow damage), tanks ignore CTC cap and wear their max stamina set of gear. Some go so far as to regem to 
[Effulgent Shadowspirit Diamond] for that fight and taking using 
[Prismatic Elixir], as well as wearing both [[Mirror of Broken Images}} and Sindragosa's Flawless Fang#Heroic.
Showing current CTC can be done by typing the following in-game:
/run DEFAULT_CHAT_FRAME:AddMessage("Need 102.4 combat table coverage. Currently at: "..string.format("%.2f", GetDodgeChance()+GetBlockChance()+GetParryChance() +5))
A longer version of that macro can total up your avoidance and net dmg reduction from dodge/parry/block, counting a block as reducing dmg by 31%, i.e. assuming [Eternal Shadowspirit Diamond] and no crit block. The net damage reduction part of the macro is most useful for paladins that aren't yet block capped, and still undervalues mastery because Holy Shield uptime isn't accounted for. Also, damage reduction from armor and from talents are separate. Modifications of this macro to make it more relevant for block capped tanks, and for warriors in general, are welcome.
/run local b,d,p=GetBlockChance(),GetDodgeChance(),GetParryChance() DEFAULT_CHAT_FRAME:AddMessage( format("Unhittable: %.2f%% Avoi
dance: %.2f%% Net Dmg Reduction: %.2f%%", b+d+p+5, d+p, d+p+5 + 0.31*b))
Before regemming gear to aim exactly for 102.4%, remember to get the relevant buffs that affect CTC: the food buff you normally raid with, 5% all stats (
[Mark of the Wild] / 
[Blessing of Kings] / Will of the Emperor). Strength converts to a small amount of parry rating, so you need your raid-buffed strength to know where you stand exactly.