I got the 234rc yesterday to use with a Nikon 7000 and Nikon 300mm f4, the total weight is around 2.3kg of this combo. By looking at the design of the 234rc head the weight centres on the 6mm bolt that tightens the head up. I would think there would be a safety margin built into the design but it's probably best to go by the manufacturers recommendations.
go for a head that's rated for your camera lens and anything else you may add in the future (grip?).don't forget the max weight handling of the monopod too. I always hold the camera by the tripod mount and have the neck strap on too, i never trust the head or monopod to take the weight of lens and camera.
Like everything mechanical is a good idea to inspect it once in a while for signs of damage that may lead to a failure.