I would be very interested in finding out myself...

This is Horus Art Tiffany Crackle subway tile with 1/16 grout line. A note about what type of grout to use per your spacing:
- If the grout line is over 1/8, you have to use sanded for the extra stability.
- If the grout line is 1/8, you can use either sanded or unsanded.
- If the grout line is 1/16, have to use unsanded.

most of the backsplashes we do in our clients kitchens that use subway tiles, esp single color and glossy, are just using the "nubs" built into the edge of the tile as spacers. This will leave approx 1/8'' grout line, but in actuality it is slightly larger if there is a slight rounding or bevel to the tile. Glossy tile with a large dull grout line doesnt look good.

The only time we use spacers for subway, is if it's carrera marble, travertine or limestone(Jerusalem Gold), because a slightly larger grout line in a usually non-white grout looks nicer.

I really think it is personal preference and the look you are going for. I do not have subways in my kitchen, but did use them in my bathroom remodel for the shower. I really wanted to see the staggered pattern of the subway so we used spacers (can't remember size...maybe 1/4th or it may have been 1/8th? it was more then the nub width) and gray (oyster gray) grout. The grout line does look fairly large but I like it. Funny thing, on the opposite wall we used 6x6 tiles in a staggered pattern with the exact same spacers and the grout line looks smaller. Go figure?

I do think in most kitchen backsplash applications people are going for a more clean simple look....white/light grout and small grout lines. I think if you are using white grout and white subways then why have a bigger grout line then what the nubs provide? But if you really have any doubt, do a test board with your tile and grout. That is the only way to know for sure...I did test boards for both my kitchen and bathroom and I am so glad I did.