This depends on the application itself. But yes, if the voltage is over the 600 you should cover it with electrical tape or Liquid Electrical Tape.
Yes, Liquid Tape is flammable but ONLY in its liquid state! Another one of our posts mentions drying and curing time. This and one other thing is very important and that is inspecting the connection you are about to use the Liquid Tape product on. A small short, friction, or heat in the cable can cause a fire if the tape has not been given the full amount of time to dry and cure. Remember, if the tape is not fully dried it is still flammable.