When it comes to grouting your metro tiles, a 1/8-inch grout line will give you a classic look, while a slimmer 1/16-inch grout line might be better for a more clean and modern look. Really though, the decision is all yours.
'What’s often overlooked is grout, the unsung hero of design,' says Deborah. 'Grout is an essential part of the whole “metro look”. The staggered grout joint is an essential part of the metro "tradition" and emphasizes the regularity and repetition of the shape, but grout joints can be narrow or wide, exactly matched to the tile, or a slight contrast.' You could even embrace an oversized grouting idea if you want to be bold.
All in all, don't just go for a default grout line. Incorporate it into your design if you really want tiles that win your compliments ever time.