You can fry an LED from reverse polarity if you put the batteries in backwards unless the light specifically has circuitry to protect against it. However, I don't think most incandescent lights are harmed by reverse polarity, which is what the SAS manual is likely referring to.

Very few lights will draw any current when off, and even then, it's usually miniscule (usually these lights have electronics for multiple levels or multiple modes). The SAS manual is probably referring more to accidental activation of the switch, like in your pocket or in a pack. Does the manual mention voltage leaks or induction? Because I don't really see that being a problem with modern lights. If the light doesn't come with a lock out tailcap, like a Surefire, you can also loosen the head or tailcap so that activating the switch cannot physically close the circuit.

Personally, I would avoid putting batteries in backwards in anything electronic.