As long as you have all your safety gear on and your work clamp is attached to the table, you won't get shocked even if you lean up against the table. The electricity wants to flow to the work clamp, so if the work clamp is attached to the table, it will suck up all the current and you won't get shocked. Also if you are wearing your safety gear, even if the ground clamp isn't attached, there will be enough insulation around your body to prevent you from getting shocked. For example, maybe you are wearing a leather apron. Or maybe you just have on a heavy pair of denim overalls. Your boots also provide some insulation. All that said, even if you do get shocked, it's usually not too bad. The open circuit voltage on these welders is something like 60 to 80 volts. Enough that you feel it, but not usually enough to do you serious harm.
When I weld, I typically put my elbow down on the table, and lean against the table for stability.
EDIT: The technician in me won't let one of those statements stand. Technically, the work clamp will not suck up "all" of the current. Electricity takes all paths to ground, in proportion to the resistance of the path. The ground clamp is, by far, the path that the electricity wants to take, but some of the electricity will flow through you to ground if you're leaning on the table--it's just such a small amount that you don't even feel it. If you were shirtless, sweaty, leaning on the table, and standing with your bare feet in a puddle, I reckon you would probably feel a tingle even if the ground clamp was properly attached and the arc was running. But you'd feel a lot more than that under those circumstances, so don't do it!