When would extra avoidance be good? (Baring you have enough threat already on this boss). Name me a situation you'd use more avoidance gear han usual and how would you do this? I have a warrior tank, wouldn't avoidance be more of a pally tank thing?
Almost anytime the mob in question hits super hard and really fast, especially dual-wielding.
Prince in phases 2 and 3
How would I do it? Depends on what gear I had available to me and the exact nature of the fight and the group composition.
Use as much avoidance gear as you can as long as...
1.) your threat isn't suffering
2.) you have plenty of health for the encounter
Having more threat than what your dps can cap is pretty useless.
like dlongest said, avoidance is best used for bosses that hit very hard and very fast.