Hello Theodore
There's nothing wrong with stacking Stam as long as you stay at 540 def.
I tanked the whole content with this rule in mind and had no problems. But i guess it may can depend on your personal role in your raid.
If you are THE designated alltime-Add-Tank getting blockcapped is probably not a bad idea. If you don't like add tanking and only do singletarget tanking you can probably live without getting blockcap. Block is not working on Satharion's Flamebreath or on the Magical Dmg at Malygos. On the otherhand, beeing block-capped can be quite good at, for example, beeing Satharion Add-Tank where you got loads of little whelps and flames to tank. At the moment every reasonable, naxx25/malygos25 equiped tank with some skill can tank anything in the content, dont worry too much and just have fun

maybe this will get more important at uludar, but who knows....
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lvl 70 / Samsara / Der Mithrilorden