I don't have marble, but...

I had used sanded grout in the last kitchen with the arabesque tiles, this kitchen I was able to do unsanded with the same tiles. The look is so much better and smoother.

just a side note, I used the expensive additive to prevent staining in the grout instead of mixing with water. It was worth every cent of the $42 it cost me. Nothing stains it all.

Thanks Beekeepers... do you know the name of the additive? It seems I am going to have to go out and buy this, so I want to make sure I'm getting the right thing.

if you want less than 1/8 grout lines, then you need to go unsanded. i'm doing 3x6 honed marble subway tiles for my backsplash and want 1/16 grout lines, so unsanded is what i have to use.

I know this an old thread but did you use unsanded or sanded?