i've been taught the same thing,
grind from the tip to the back so all
the grind marks are radial.
i use a belt sander, its just as easy.
be careful the point doesnt catch.. or
point it the other way. :)
i'm guessing the mushrooming would only be a problem
if you're using your punchmarks for delicate layouts.. where,
for example, a steel rule could get hung up when sliding it around.
in the end, as long as its 'pointy' it should be ok.
unless we're talking about transfer punches.
i sharpen those in a lathe.