The scissor jack that comes with the car has a slot in it.
This means that it doesn't damage the seam under the sill when jacking at the marked points.
If you're going to use another jack, and jack at these points, it's best if you can use a slotted bit of wood or something similar (and secure) to avoid damage.
I speak from experience, my front wing as popped a bit out of alignment when a mechanic just trolley jacked with flat cup straight onto seam.
Of course, if you're stuck, don't worry about it too much....and as said, make sure it doesn't fall on you. There was a guy killed working under his car a few miles from me recently when it fell.