You just have to use some common sense. If the floor is rotted, then dont do it. If the nuts not welded OK, then dont do it, if you loop welds are crappy, then dont do it, If you car has an engine and everything else on it....I wouldt do it, If you are gonna be standing under the car welding something like the guy on the other pic, then use extra support for safety, etc.

Taking a walk outside and crossing the street is also dangerous just so you know, a car can run you over. Just be sure too look both sides before crossing....duh!