Mark:
I don't want to flog this to death, so I'll just try once more to get across what I was asking about: if the welding machine's outputs (either one) are not connected to chassis, then if a short in the machine were to happen between the torch output and chassis ground, the work clamp and anything it was connected to (like the table) would move the full output voltage off of chassis ground, all without tripping any breakers or shorting the machine's output. But the work and table would be dangerously hot. Undoubtedly this is not a common occurrence, but certainly possible. That is why it seems to me that the work table should be earthed for safety. This could be done with a wire from the welder's chassis to the table.

From the other posts I gather that grounding the work table is not common practice. But in the unlikely event of the wrong kind of internal machine malfunction, it might save you from a bad shock.

Thanks again for the thoughts and comments.
Craig