Trying to gear up... Tia's Grace

So I used to play a druid back in Vanilla and BC- not much in Wrath before quitting for a while. Came back with friends, and we're pretty casual.

I don't think I'm going to be doing raids anytime soon (just a few friends in a personal guild, again casual).

I think I'm doing alright gearing wise, hit 85 about a week ago and gone through some successful heroics (in fact, some that were very enjoyable).

In any case, the other night I apparently committed a sin by rolling need on Tia's Grace. I had read that Tia's Grace is one of the best pre-raid trinkets for Blood DK's due to it's large amount of mastery and the agility proc giving us about 1% dodge. The Porclain Crab and the Tol Barad trinket being the only two better pre-raid.

Am I mistaken? It caused quite the upset- I was told I should go for the aforementioned Crab and reforge the dodge to mastery.

Am I missing something? Why would a trinket with dodge and a mastery proc on it that I then reforge to mastery be better than a pure mastery trinket with a 1% permanent dodge proc?

And further, why would Tia's Grace be such trash for me, and Throngus's Finger which is pure avoidence be so superior when I reforge it, too, to mastery?

Any other advice on my gear would be welcome too- I'm trying to craft the epic chest from BS (two more Chaos Orbs to go) and working on Hyjal rep for the belt and back.

I also know I need to reforge ~50 dodge to rating to even them up.

Thanks for the help-

And no, I didn't win (let alone take) the Tia's Grace.

I used Tia's Grace myself until something better came along. To this day I've never even had the chance to roll on Porcelain Crab. (IIRC was using Tia's Grace and Impetuous Query before I got my first epic cata trinket).
Take upgrades as they come. While Tia's Grace is decent for DK Tanking, and may be more effective for some other classes, an upgrade is an upgrade. I would not consider what you did in any way wrong. What you could have done better is announce your intent to roll on the trinket at the beginning of the dungeon - I would more see this as an issue with expectations than doing the wrong thing.
As a DPS DK, I roll greed on that(with a hunter in the group), as I want it for my tank offspec.

On my Troll DK tank, I rolled need on it(also with a hunter in the group), because mastery and agi(for dodge) are awesome tank stats.

We all have to share gear- just because some people don't understand or accept that a piece can be used by others doesn't make you wrong for needing it...

... but for the love of all that is good and holy, know what you are doing before you roll need on anything. Know your class and don't be greedy, and the game will be better for everyone. (That's a general message to everyone, not directed at OP)