I have the Subway Ceramics Avalon / Minas with a medium gray grout. And a Shaw's white sink, if anyone is considering that, by the way. (My contractor could have ordered it directly, but at his request, I went through a tile store, and by the time everyone had put their sugar onto it, it was OBSCENELY expensive.) I'm not an installation expert, but at least with the Subway Ceramics, the installers use little plastic spacers between the tiles (I think some others have their own spacers?) when they put them up, they don't do it freehand, so how much grout you get is a function of 1) the limits set by the curvature of the tile and 2) what size spacers they use. So you might be willing to go a little darker with a smaller grout line, or with a handmade tile that refuses to go close together, you might want to stay light. The traditional color is close to concrete. But most modern people don't seem to like that, so do what pleases you!

kegrant- I too would love to see photos and do post on what you finally decide. I too have simply white cabinets (ikea lidingo) and soapstone counters and am leaning towards white subways with white or possibly silvery gray grout (DH's preference) but everyone who looks at my kitchen says you want to add more white?!?! I am pretty confident I will love the look, but photos are always better. Best of luck to you in your search!

Trying to decide between medium gray, light gray and white grout. Advice?

Honestly, I think it's a personal preference thing. I've put in subways in various rooms for over 10 years and I always did contrasting grout b/c I like to be able to see that distinction and highlight the pattern. To me, white grout looks blah and artificial somehow. But I've had tilers who have argued vehemently with me that I'm "doing it wrong" when I ask for contrasting grout. I like a really dark contrast, but like jannicone says, the darker the grout the more perfect the tiling job has to be. I've always done a light or medium tone contrast for that reason. Looking at your pics, I think contrasting grout would tie in beautifully with the darker lines of your counter. I think it would also help to break up the all white of the uppers with the tile. Nothing drastic, just a light to med. grey - but that's just MHO.