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With regards to SG versus SS:
No the 4% parry from SS is not subject to DR. However, that does not make it categorically better than SG. It really depends on your gear level as well as your spec.
Gear: I believe SG scales with stam from gear as well as your base stamina. Therefore the higher your stamina, the more benefit from SG. Also, as you progress in Ulduar, a lot of the gear actually has less defense on some of the BiS items and more other stats (dodge/parry/stam/expertise/hit/etc...)
Spec: I personally think the SS rune is more conducive to an unholy spec. Frost generally likes a mix of stamina and avoidance but does not gain any specific benefit from avoidance as does UH spec (higher BS uptime). However for certain fights, avoidance makes sense due to fight mechanics. This is especially true if there is some sort of stacking debuff on hit (i.e. Add-tanking on KT, Kologarn, etc...)
In general, as you progress in gear level in many ways it makes more sense to stack stamina instead of avoidance (especially as a blood spec but even as a frost spec imo) in order to boost your EH and give your healers a larger cushion for those big hits Ulduar mobs are capable of.
At the end of the day, it really depends on what you're primary role will be in raids. As any form of MT, I would definitely consider stacking stam (and therefore the SG rune). As an OT, I really believe either/or are viable.
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