Your instincts are good on this matter. Since the plasma work lead is actually positive (+) you do not want to hook the table up to your house grounding rod! When the work lead in connected to your metal, the table is charged positively.
If you want to ground the table, it needs to be to a different (separate) grounding rod, that is what people are usually referring to.
If you did put in a separate grounding rod for the table, it might be wise to check to see if you are getting continuity between that ground and your house ground. If you are, that will not work.