The points arc on opening. There's no difference in where the arc occurs. For timing with a strobe or static light, contact condition makes no difference. If you set the gap with a dwell meter, again no difference. It makes a difference if you measure the gap to set dwell.
You have to dress the points before gapping because you need to measure between relatively flat surfaces. You file off the "bumps" so you aren't measuring inaccurately. The "valleys" aren't as important, but getting both surfaces flat gives a larger contact are as long as they touch flat to each other.