uH, you can do that but its not good for ur machine. you want it on the same piece of metal your welding on but you dont want it closer then 2 or so feet. You get an enormous amount of resistance travelling through the ground if you do. Also weld away from where you place your ground.

Im not saying its not possible iv done it before its just not good to do it alot or for more then tac welding.

as for stress points as long as you have a large surface area most mig welders dont heat the metal enough to cause it to have serious stress issues. (i assume u have a mig machine) I welded my sliders vertical and i havnt had a problem, they are sitting on 1/4"x4"x5" plates.