If a driver is physically threatening you with the car (i.e. trying to run you off the road) or driving in a way that puts you in serious danger, then I would definitely report it to the police.

Remember what happened in the Dr. Thompson road-rage case in LA? Dr. Thompson's aggressive actions against cyclists sadly ended in serious injuries to two cyclists. His past threatening actions, which were reported to police, helped convict him of felony assault with a deadly weapon.