The most important thing about tightening a banjo head is to get the tension even across it. You are more likely to rip a head if the tension isn't even.
Using a torque wrench sounds too much like an April Fool joke to me. The tension should be spread evenly between the adjusters, but the overall tension on any 2/one of + the n/5(excuse 2 year old adding extra numbers) - any one of the nuts will depend on the number of nuts, the skin diameter etc. Some of these are standard on a modern instrument, but as mentioned above, can be influenced by small additional factors.
I favour pinging the head with a flick of the fingernail to see if it rings, and if the ring suits the tone of the banjo.
The other factor is the amount the bridge sinks into the skin when the banjo is tuned to correct pitch. Too deep and the instrument will not tune correctly, so the head needs to be tightened.
The difference between a plastic head and a vellum is that a natural skin is neither a standard thickness or composition. Too thin a skin will break at a lower tension, but too thick does not ring out as nicely.
Quack!