Do I need one? Yes, if you can afford one.
If you can’t afford one then make sure your DP knows HD well and can provide the technical quality you need.
Shooting quality HD is a bit harder than shooting film. DITs role is to ensure the quality and integrity of recorded signal. DIT position is similar to video utility position in a SD multiple camera shoot but often goes well beyond.
A knowledgeable DIT can work closely with the DP and post production colorist in creating a custom look for the production. A DIT can get inside the camera and adjust things the rental company would rather leave untouched. By doing so the camera can be forced to respond to light in a unique way and create the specific color you want.
If you can’t afford a DIT but are still concerned about signal integrity hire a camera assistant who can read a waveform monitor. Make sure you are getting all you think you are getting. Monitors on the set can be very deceiving. Trust a measuring instrument only.