Tony is spot on, you dont 'need' the RPM wire connected. However if you are using the telemetry features of the iKon2 you can get RPM to your transmitter via the telemetry link. If that's something you want to implement you would want to connect the RPM wire.
The other thing the RPM is useful for is vibration analysis. The iKon2 can tell you the source of vibration based on comparing the frequency of vibration and RPM of the spinning parts of the heli. To do this it needs the RPM signal.