Nope, My grandmother never soaked bread used for Meatballs in milk. I
have no idea myself what the purpose of using milk does to meatballs?
Maybe someone else can comment about this? To me, this sounds like a
yucky ingredient to be adding into the meat/mix.

I've never heard of other Italian friends/families adding milk to
Meatballs?

My grandmother probably made meatballs for 70 years. She never measured
anything. It was always a pinch of this, and a pinch of that.
I reckon she cooked with love, rather than cook from a book.

I must've watched her make Meatballs a 1000 times. While mine are good,
they could never duplicate, or compare to hers. I've always asked
myself, why is that!? :-) Mark D.