I tell students a minimum of the thickness of a nickel. (On the other hand, Rafael Martinez has been known to sand a brand new tip down to maybe the thickness of a dime on the sides. Crazy.
Yes. Leave it alone. Only if it gets shiny and stops taking chalk should you rough up the surface and without removing leather. Of course lots of players chalk really badly -- members of the Borer Tribe.
When it comes to chalking I am a stroker, not a driller, and I rarely have to do anything to the tip surface. It seems the intrinsic abrasiveness of the chalk, being stroked on, does what's necessary.