The conventional wisdom is that it should be grounded.

If the workpiece clamp is connected to a grounded table AND there is a bit of "voltage" leakage through the welder into the output leads then attaching the leads to a grounded table bleeds off the leakage. Note that such leakages are usually high-impedance leakage which means they can't supply much current so they will drain off without blowing the breaker. These leakages can give you a tingle.

A possible problem with a grounded table is that you now have a large item at ground potential that you are very likely to come in contact with. Now consider a welder that has a significant fault in it such that the workleads have a connection to the line supply voltage. You touch one of the leads - nothing happens since you have dry boots on. Now, you lean against the grounded table - zap!. If the table wasn't grounded you wouldn't have received a shock.

Overall, it is like, as has been mentioned, the seatbelt argument. Sure some times the seatbelt will be the culprit but the vast majority of times it is the safer way.

If you have a stick welder and you touch the stick and the workpiece clamp you will feel a tickle with bare skin or wet gloves since the OCV is about 80V. Note that the welder will work whether or not the table is at ground potential.