533 is fine. You can drop down to 536 with just the 15 resil 30 stam shoulder enchant. The way to calculate is by mousing over the stats in your character sheet. Mouse over defense and see how much crit reduction it grants, then do the same for resilience. Does it add up to 5.6% or more? If so, you're fine. If not, switch things around. I'm betting you're fine. That's the best shield in the game next the heroic Icecrown Glacial Wall, I'd get it if you can.
I've been using the wrathful shield for over 2 months now. Definitely uncrittable as low as 522 defense. Like Dreador said, just add up your crit reduction chance from resil with your crit reduction chance from defense >=5.6% = golden for pve.
I just want to throw one small thing into the discussion (I'm using the pvp shield too, but i'm still above 540 defense). If you fall below 540 (or 535 for heroics) defense you will not be crit due to the resiliance but you can be dazed again if struck from behind. While it's not a big deal for raids where you shouldn't be hit in the back anyway, it's really annoying when speed clearing heroics.
Just keep something in your inventory to switch in when you are doing your daily heroic and you will be fine.
I do believe the Heroic Glacial Wall is the only thing better than the Wrathful shield, but it's a tight comparison. The Normal mode version is not as good as Wrathful or as good as the Heroic Neverending Winter.