Honestly you're going to have enough trouble keeping the marble white much less the grout - seal it all!! THREE TIMES! lol Seriously though I've heard horror stories about smoke and wine stains in the kitchen. Sealer every year is about the only way to avoid stains.

I've been lusting after the white marble backsplashes myself but after spending half an hour scrubbing the white grout on the marble in my shower yesterday, NO MORE! I went with whisper grey on the floors in the shower and bathroom and they show dirt far less. If I had to do it all over again I'd have gone grey all the way.

Good luck!