0 is the ideal amount of accuracy to have
This. Accuracy is useless for a tank. You shouldn't have any problems holding aggro, and if you do, that's what taunts are for.
For Vanguard/Powertech tanks in particular, Accuracy doesn't even help your DPS very much. The only attacks you will ever miss with are some ticks of Hammer Shot and Full Auto (if you even use
Full Auto) and the occasional High Impact Bolt. If you were wanting to boost your DPS a bit, you'd almost certainly do better with Crit, Surge, or even Power, rather than Accuracy.