Found this on the official forums,believe it or not o.O
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+Hit Caps for Bosses
Hit rating, expertise rating, and spell hit rating are one of the best ways to increase DPS against raid bosses. As raid bosses are considered level 73, you have a much higher chance of your attacks missing than normal. Additionally, hit rating tends to be a "cheap" stat when gear is itemized, meaning that an equivalent amount of hit rating usually does more for your DPS than the same amount of critical hit rating.
However, eventually you have enough hit rating to never miss the boss, and accumulating more hit rating is unnecessary. This is usually referred to as hitting the "hit rating cap".
Spell Hit Rating
Base Spell Miss: 16%
Spell Hit % = spell hit rating / 12.6
Spells actually have a 17% chance to miss, but you cannot eliminate that last 1%, so the effective cap is 16%.
The following is the amount of spell hit rating you need to minimize misses for raid boss fights, depending on your class and talents.
Mage
Base: 202
w/ Elemental Precision (Fire/Frost): 164
w/ Arcane Focus (Arcane): 76
Warlock
Base: 202
w/ Suppression (Affliction): 76
Note that a warlock might still want extra spell hit rating for Shadowbolt and Soulshatter, in which case Suppression becomes redundant.
Priest
Base: 202
w/ Shadow Focus (Shadow): 76
w/ Focused Power (Smite, Mind Blast, Mass Dispel): 152
w/ Shadow Focus + Focused Power (Mind Blast): 26
Druid (Spell)
Base: 202
w/ Balance of Power: 152
Shaman (Spell)
Base: 202
w/ Elemental Precision: 126
w/ Nature's Guidance: 164
w/ Elemental Precision + Nature's Guidance: 88
Paladin (Spell)
Base: 202
w/ Precision: 164
Other Factors
Mage/Priest/Shaman Draenai Aura: -12.6 spell hit rating needed
Shaman Totem of Wrath: -38 spell hit rating needed