Actually, this is a bit wrong. Special melee attacks (re: everything but your basic attack) have a 100% chance to hit. The reason that you don't always hit is because operations bosses have a certain amount of defense (6-8%, iirc) that needs to be counteracted.
Not true. If you read the linked post above (Tam's one on Accuracy being harmful for tanks). Special Melee attacks have a base 95% accuracy (his examples being Double Strike and Spinning Strike). Presumably to offset the base 5% Defence chance.
For a Guardian that list is a lot longer: Overhead Slash, Sundering Strike, Master Strike, Leap, Throw, Dispatch and Riposte. Yes, I have seen every single one of those "Miss" (or "Dodge") on an Ops boss. For reference I have 3% Accuracy from my tree and 1% from Legacy. Giving me a base Melee Accuracy of 94%. I have none on my gear.
Agreeing with Kitru, I'd probably add in Strength before I added any accuracy (swapping Guardian armourings for Might ones), but I'd add accuracy before adding Power, Crit or Surge. That said: I personally don't have an issue with threat so its a trivial argument.