is accuracy useless for a powertech tank ? why ?
The only attacks you (should) use as a VG tank that benefit from Accuracy are High Impact Bolt (15 sec CD, hits hard but not that important because of the long CD) and Hammer Shot. Everything else (Energy Blast, Ion Pulse, Explosive Surge, Stockstrike, Mortar Volley, Pulse Cannon) are Tech attacks and, as such, gain no benefit from accuracy (since they already hit 100% of the time). HiB and Hammer Shot are such a minute portion of your total threat and damage generation, that you'll see such tiny returns from stacking accuracy that it's functionally worthless. You're better off stacking Power if you need more threat, but, since you're a tank, you're better off just taunt spamming to solve any potential threat problems and cramming more mitigation into there.