They are designed not to be tampered with.
When they are tampered with, you can sometimes tell, because the numbers dont all line up anymore. And ive heard stories of people winding back an ODO (not in a skyline), and then it breaks down a few thou kms after that.
When buying a second hand cluster, you could get lucky and get one with less kms, or unlucky and get one with more kms. As long as you have a log book since you owned the car, showing your services, etc - you shouldnt be too worried about the extra kms.