torque wrench and piston question

FWIW,

In the scheme of things using a torque wrench is the least accurate method to tighten a threaded fastener (tightening by feel being the only less accurate way). Bolt stretch, then strain guages followed by torque to yield are the most accurate way.

That being said the best thing a torque wrench can do is be repeatable since the actual clamping force in the joint is going to vary by as much as 40% due to friction variations. On a click type wrench the best way to ensure it's repeatability is to back it down to zero after each use.