Most plate tanks understand their stat priorities; after their hit caps it's usually reforging and gemming Mastery, avoidance or Haste.
While the avoidance geared tanks are generally fine, the tanks who have avoidance as a secondary stat like Mast DKs will end up dumping excess accuracy stats into Parry via reforges without consideration of Diminishing Returns.
This usually happens when someone uses AskMrRobot; the sly little bag of bolts puts Parry over Dodge in stat weights meaning that it will ignore any attempt at balancing your DR.
This causes you to miss out on a free increase to Total Avoidance.
If this sounds like something that could affect you as a tank then check out:
--REMOVED--
It goes over what exactly is Diminishing Returns and how knowing about it can benefit you as a tank.
Balancing Avoidance and DR
Due to Parry double dipping with SoB stacks, there is no real reason not to prefer it to dodge. The resources you gain with parryhaste (your melee speed increased after a parry) outweighs the resources lost with properly balancing DR.
From sticky.