I would keep the seam close to the body line, however you do it. The tack welds on a flat surface far away from anything rigid will just distort the hell out of the panel. If the welded seam is close to that body line, the inherant rigidity will help keep things a lot straighter.
I don't have pic's showing just before I filled and primed, but I called it good when I had a full penetration tack every 1/2" of less. You just got to move around a lot with the tacks so you don't overheat any one area.
Rene