If you don't have enough health for YOUR healers to react before you die, whether that be to spike damage, an unavoidable mechanic, whatever, That is the point where you need more stamina. It doesn't get more complicated than that.
Outside of that, You should be just fine gemming haste or mastery, whichever you prefer.
In Heroic content, specifically on certain fights, and in 25m, it is more common to see tanks gemming stamina more heavily.
Askmrrobot isn't really a good site, depending on how it is used. Using presets most of the time, especially for tanks is ineffective. I wouldn't even recommend dps use this site unless they know their personal weights.
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