You do not need the instrument rating, however you will have to do the 5 hours of instrument training. It is part of the Part 61 requirement. This 5 hours is oriented toward helicopter instrument flight. The helicopter instrument add-on is 15 hours and that 5 hours counts toward your instrument add-on. Some parts of the world do require a helicopter instrument rating, like the North Sea. Plus some contracts require the pilots to be instrument rated even though they are flying a contract that is a VFR only contract, like seismic.