Hi,
Just answering your question from a few days ago. Me personally sand them because the steel gears seemed to mesh perfectly and have very little notchiness. I noticed all the plastic gears (especially the two large ones) are quite a bit thicker than the steel ones so by sanding the plastic I figured it would make the plastic gears less notchy.

I know when the white Tamiya gear diff came out Jilles had a video on sanding the plastic gears as I believe the team was having issues with them lasting, sanding the gears seemed to help that.