Maybe I am just not normal, but I keep a high avoidance set and a high EH set, and I just swap out pieces as needed.
As far as gemming goes, in my normal boss tanking gear, almost all my pieces are gemmed 15 stam, and my avoidance in that gear is 59%. I don't gem it that way because I think it is the superior way to go, but because of the socket bonuses typically aren't worth it. I have found through experience that the default avoidance on gear is typically more than sufficient, so I don't have to worry about how I gem at all.
Of course I am only tanking BT, but so far gemming all stamina has had no ill effect on me at all. I wouldn't say you want to do that all the time, but it certainly won't break you in BT, as the gear provides plenty of avoidance before you even gem it typically.
Maybe things will change in Sunwell. I expect avoidance to play a larger role there, but honestly it has never mattered so far. It seems perfectly ok to gem how you want at least through BT content. I know in Kara I gemmed to get uncrushable, and now I typically gem according to socket bonus worth. Gemming to fit the situation feels like the smarter way to go.
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