Ok to bring it down abit for some ground rules about what avoidances to stack.
Defense vs Dodge:
The value of defense for mitigation is very close to that of dodge. Defense will give slightly less avoidance then dodge but it will give you block and crit, along with a better kind of avoidance*.
So defense will be slightly better then dodge if you can use block rating or if you need more to reach uncritable.
Dodge will be slightly better if you cant use block rating and well above uncritable (so it doesn't cause problems when swapping gear)
*Better kind of avoidance in the sence that parry gives parry haste and miss works from behind and while stunned/feared.
Dodge vs Parry:
With talents and base values you should have 10% dodge and 10% parry, atleast for warrior/pala these are not included in diminishing returns.
So for it to be worth it gemming for parry you will need to have more
10% parry: you need more then 20% dodge. (base)
15.6% parry you need more then 31.2% dodge. (uncrittable)
17.5% parry, you need more then 35% dodge. (normal stats)
20% parry, you need more then 40% dodge. (high stats)
And the formula:
Parry is worh more then dodge if:
2x Dodge% in tooltip > parry% in tooltip
As you can see it's highly unlikely that you will ever care more for parry then dodge.
Note that doesn't mean that parry is a bad or wasted stat in any way on an item, due to the stat allocation on items having defense, dodge and parry on an item will always be better then just defense and dodge.