I look at it more from an art perspective and color limitations.
The color of gray for me, would depend on the content of the art.
Art with a lot of light gray can be great. Provides a good bright underbase....and, you can use that as one of your colors. In cases where the gray color is actually being used in the art, I go up a Pantone number, knowing that it's going to darken more on the shirt.
Art that has a lot of darker shadow tones can be under based by what's considered a darker gray. Not too dark, but a middle of the road gray or slightly darker.
This makes it much more easier to hold deep shadow tones.
Most often, I prefer to use a medium gray that leans on the light side.
For this, I can split the art in the middle. For example, I can use the top white and even colors to make lighter colors and pastel whites. I can use a dark blue or black or even red to lay over the medium gray to make good shadows. Here in this case, I would be using more black than I would if using a dark gray.
It's always best to try and use as little black in your art as possible. I tend to mix color with darker color predominantly and add in a bit of black to beef those dark areas up more.
It's rare, but there are times when I might even set the art up to have a black ink flashed and print under some other grays, if possible. Wet on wet. That makes a more rich and soft grayscale look.
Gray inks like that on gray tees can be awesome.
These are just different ways to skin a cat.
