Well for my purposes different encounters require different gear. For example at naxx loatheb I use block gear.
Being capped is a good thing but depending on a fight if there's lots of magical damage (e.g. sartharion/malygos) I'd use more stam gear than worry about being block capped.
Def cap is a must but block cap is a bonus since no crushing blows are happening. I have multiple sets I switch from block cap, avoidance set, max HP set and so on as I like to be *prepared*.
EDIT: Just to say if the fight you're about to undergo can be done with being capped do so but if you lose too much avoidance by doing so don't (e.g. being hateful tank at patchwerk 25man).
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