When you’re welding in general, and using any table, it’s safe and routine practice to clamp your ground to the table and place your work material on the table. It’ll allow the current to pass through, and almost every welder does this every day.
I’m also making the huge assumption that your clamping spot and material on the table are clean and don’t have dirt, paint, or other things on them. Rust, mill scale, paint, oil, or slotted/sectional tables with poor bonds add resistance.
Having said that, when you clamp to any piece of metal that’s in contact with the one you’re welding, instead of directly clamping, there’s going to be some energy loss. With a clean table and good contact, we’re talking about like 1%. But if you are clamped very far away, to a dirty table, you might see a 5-10% voltage drop. If you’re using MIG, you might not notice. But with a certain process on certain materials, like TIG on thin aluminum, you might see erratic arc starts, arc wander, or poor cleaning action.
One way to get around this issue (but only for steel) is to replace the clamp on your welder with a magnetic ground clamp. I’m a big fan of the Magswitch 300 Amp Magnetic Welding Ground. I leave it attached to my welding table, but IF I want to move it to my workpiece, it’s super easy and you don’t have to worry about being able to literally clamp since it’s just a magnet.