First, let me say I'm asking in this location since this thread was mentioned as an important 'must read'. The question came up as I pondered a (now locked) thread, started by a poster who didn't seem to articulate his thoughts very well and also didn't display an appropriate level of consideration to the suggestions of other posters, found here:
http://www.failsafedesign.com/maintankadin/viewtopic.php?t=19903
Anyway, the question of that failed thread was this: "Why do we need 540 defense when we could stack dodge?" Which obviously was shot down. You must have 540! You must have 540! Were by and large the general responses. (Also, "you've refused to read any of the material we've told you to read!" made a strong showing.)
However... I realized the question/idea has value if stated in a slightly different context. Please bear with me.
Let's assume that a player has 540 defense *AND* is block-capped - this player's combined miss/dodge/parry/block values all come out to over 102.4%. If that user was able to drop some defense rating, thus bringing his or her total to under 540, *YET* that user was able to replace the lost avoidance and mitigation with some other rating (either block, dodge, or parry), thus bringing him/her back above the block-cap (102.4%), then he or she would still be uncrittable.
In the lone case where you can magically reach the block cap without 540 defense, then no, you do not actually need 540 defense to be uncrittable.
Of course, you can only use all of your avoidance when you are facing a mob - you can't dodge attacks from behind, for example. So you could still be crit if you were turned away from a mob and had less than 540 defense.
So we add the final addendum: Assuming you are always facing the mob, and provided you've found some way to reach the block cap with less defense rating than would take you to 540 defense (note that in all likelihood such gear does not exist), you do not need 540 defense to be uncrittable.