Below 540 def is fine
Tank in 535 def, run Recount.
I tried this in 10-man Nax and got 0 crits on any bosses. Fluke? Boss-specific?
Or maybe the 540 thing isn't so absolutely necessary, just a guideline?!
Maybe I'm late in finding this out.
No, it's not going to stop a decent group from having reasonable success. Many of the first tanks to clear Naxx near WotLK release were not at 540 defense.
However, you are walking around with a golf club in a thunderstorm and claiming it's not dangerous because you have'n't been killed yet.
Defense is the best scaling avoidance stat currently in the game, not having enough hurts avoidance as much as just allowing a small chance to be crit into oblivion.
Random number generator is random.
At 535 defense you have a 0.2% chance to be crit, and only by a boss. So yeah, chances are good you'll never be crit. You still want to get that last 5 defense to make sure, and it's the best avoidance stat anyway.