For trash, put on threat stats unless it's a tougher group of mobs (say the groups of abominations in Hyjal). This means more +hit, and more +expertise (although the expertise cap for trash is 20), and more shield block value. For tanking bosses it's gonna vary.
If you're up against a super hard hitting boss and threat isn't an issue?
Probably use Moroes Pocket Watch and Commendation of Kael'thas
Up against hard spell casting boss?
Probably us Commendation + Stam Trinket
Up against threat sensitive fight?
Probably the best trinkets for each type of sitation are here:
Health/auto "Oh Shit" : [item]Commendation of Kael'thas[/item]
Expertise: [item]Shard of Contempt[/item]
Hit: [item]Icon of Unyielding Courage[/item] or [item]Romulo's Poison Vial[/item]
Dodge: [item]Moroes' Lucky Pocket Watch[/item] or [item]Shadowmoon Insignia[/item]
Defense: [item]Scarab of Displacement[/item]
SBV: [item]Gnomeregan Auto-Blocker 600[/item]