QEP makes a round spacer they call the Bullseye, which lets you clearly see the place where the tiles meet at the corners. It has two sides, with a different grout-joint thickness on each side.
Its design prevents you from pushing it too deeply into the grout joint, and it is easy to remove. It's not available as a T-spacer for use with running-bond pattern on subway tile, yet with two quick snips it can be quickly turned into one. You end up with a horse-shoe shape, with the vertical spacer on the top of the arch and a horizontal spacer near the end of each "leg". And the leftover piece is not wasted. You can use it as a spacer where the walls meet. In fact, QEP includes a small bag of pieces similar to the leftover piece, for just that purpose.
I think it's a better solution for subway tile than turning a standard + type spacer sideways, or using three bar-shaped spacers, or using a T-shaped spacer that has to be pried out with a tool; the modified Bullseye spacer supports the spacer in the vertical joint and it's all one piece. The QEP Bullseye works better with tiles that do not have a perimeter ridge.
I phoned QEP to ask if there was going to be a T-spacer version, and was told that subway-tile installations are too small a slice of the market, so probably not. Too bad, as it would be an easy mod for QEP to do, and they could "corner" the subway-tile-spacer market