Stem direction of grace notes when main note is stemDown

[Knute Snortum]

I'm confused, which often means I'm missing something, so bear with me.

I have always thought that a grace note's stem is up no matter what.
But LilyPond's default is to engrave a grace note's stem down if the
main note's stem is down:

{ \voiceTwo c'4 c' c' \acciaccatura d'8 c'4 }

...which forces one to do something like this:

{ \voiceTwo c'4 c' c' \acciaccatura { \stemUp d'8 } \stemDown c'4 }

Even then, the slur goes from the head of the grace note to the tip of
the stem in the main note, when I think it looks better when the slur
goes head to head.

LilyPond has engraved grace notes down stem when the main note is down
stem since at least 2.18.2, but this seems wrong.  Shouldn't the
default be always stem up?

--
Knute Snortum