It depends on what surface is in the call. On a glass, alu. or copper, I go horizontal only. On a slate I've done both horizontal and circular, if you want the striker to grab better go horizontal, if you like a smooth slick surface go circular. A smooth surface gives the call a little higher pitch but is harder to play.
For my strikers I twist/grind into the sandpaper so that it is a rough tip, it's grabs easier. A smooth tip plays great on a rough call, but as the surface starts to get glazed it skips to much.