It's a pretty simple chart.
Are you dying/are healers having issues healing you?
No -> Parry/dodge for more DPS
Yes -> mastery to keep from dying.
Through most of heroic content my MT and I (we're both prot warriors) were able to go parry/dodge for more dps... until we got to heroic siegecrafter who hits really really hard, and then the heroic paragons hit really really hard, and supposedly so does h-garrosh, but we're not quite there yet (oh and fyi this is 25 man). So we're back to Mastery now for survivability.
As far as DR on parry/dodge... just go full parry, you pick up dodge where you can, maybe matching yellow sockets with parry/dodge gems. The reason for this is that even though this isn't the "maximum avoidance" you're not going parry/dodge to be a better "tank" as far as absorbing damage and being easier to heal. Anyone who tells you that is lying and trying to justify meter padding. Avoidance builds are for increased DPS. Period. The reason it does so much more dps is because parry = hold the line proc= beast revenge crits.
And since Riposte is 75% of avoidance % gain... well... you could balance out dodge and parry... but you're talking about like maybe a 3-5% difference AT MOST which would lead to a 2-4% increase in crit AT MOST. Not that big of a deal for a tank, and then you're dodging more than parrying so you're losing out on HTL damage.