Balancing the two to a certain ratio is certainly the best train of thought. However, there's a factor that makes this pointless that I overlooked myself at first until I picked it up at these forums. The dodge and parry you get from Defense Rating counts toward the DR also, and it's giving just as much parry as dodge. Thus, any tankadin that is uncrittable most likely has their ratio skewed far in the Parry direction. The more dodge you stack, the closer you are to that perfect ratio. I don't know the exact numbers, but it's true that if you have enough dodge, parry becomes more valuable. I'm pretty sure this is more practical with Death Knights as they have more favorable DR ratios for this.
Similarly, you're better off getting Defense Rating than Parry, as it gives plenty of parry.
I'd love to see the balance of 1 point each Dodge Rating, Block Rating, Parry Rating versus three points Defense Rating. Defense also grants Miss.
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well the amount it cost me it's staying there, sub-optimal or not.