Dropping your Defense skill to 490 is generallly a good idea, one thing to keep in mind though is that high Defense also reduces the enemy's chance to hit you. Just so you know.
"Defense is a skill that helps prevent physical damage by reducing the chance to be hit or critically hit. Additionally, it also increases block, dodge and parry. Mobs receive 0.1% per point of defense. Mobs get an additional benefit from defense of Glancing Blows versus players.
Each point of +Defense skill adds 0.04% to the chance to be Missed, to Block, to Dodge and to Parry for players. This means +25 Defense will grant you an extra 1% Miss, Parry, Dodge, and Block. The formula is constant across all classes. It further decreases the chance of receiving critical hits from any level attacker by 0.04% per point that the target's Defense skill exceeds the attacker's Weapon Skill. Please note that a decrease in your chance to be hit (like the tooltip says) actually increases your chance to be missed. However, all of these chances are based on an opponent of equal level. The rating difference between the attacker's weapon skill and the defender's defense factors into the true chance for miss, parry, dodge, block, crushing blow, glancing blow, and critical hit." - Defense - WoWWiki - Your guide to the World of Warcraft
Defense may not be the best way to go for avoidance, but it covers all the areas if you don't have items that are more avoidance specific. That being said, if you would like to drop some Defense, I suggest getting some of the following items:
[item]Brooch of Deftness[/item]
[item]Girdle of the Fearless[/item]
Yes some of the above items have Defense, but its in a large chunk. This will allow you to take up more items for pure threat or effective health like the [item]Bonefist Gauntlets[/item]. The Expertise on the items also helps with avoidance seeing how Expertise decreases Parry, as well as being Threat from reducing Dodges. Again, Defense isn't bad after 490, but its not the best way to increase specific avoidance areas. Hope this helps some.