The main thing is, like I posted pages back, to have that balance in your gear for what you're doing. There are other and probably better places to stack avoidance. Targeting gemming as a place to get avoidance easily overlooks some other key places (mentioned in this thread already by others, but I'll repeat - enchants, trinket slots, gear choices).
again:
If you are dying to burst damage before healers can get heals off (e.g. one /two shotted), add more stam. If your healers are running out of mana, add more avoidance+mitigation. If you are running out of mana yourself, you probably have too much avoidance+mitigation.
example:
When what you need to do is survive tanking Brutallus before you have any of the loot from Kalecgos, there isn't much room for regemming for avoidance if you want to survive Brutallus' burst nightmare scenarios.
Ultimately you have to be able to survive unavoided hits particularly when they happen in combination or strings.
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Mavrix
BE Tankadin
Dalaran
vicis pro factum
http://www.wowarmory.com/character-sheet.xml?r=Dalaran&n=Mavrix