This is one of the few things in the JAR's that are pretty simple.
According to JAR, you can log as flight time, the time from when the helicopter moves under its own power for the intention of flight until it comes to rest at the end of the flight and its rotors have stopped.
Not excactly rotors running time since the machine can sit for a prolonged time before flight is commenced, say during pre-flight checks. Usually, the helicopter is not run for long after the flight.
During aerial work where you land and take off all the time, I think it is not wise to have the pilot to calcluate the ground time off his flight time. The difference between technical time (skids on time) and flight time can be some and you as a pilot log the flight time according to the relevant regulations while the helicopters technical tie is logged according to different regs., not to be confused.
The pilot is (at least he is expected to be) at the controls all the time and the helicopter is handled as in "flying" conditions while rotors are running. Thats what the JAR's expect.