I use the Luminosity ramp in the color picker when I lay out a greyscale color pallete. I started paying less attention to the default "greyscale" pallete in regards to correct tonal value when I noticed that the values just didn't seem correct to me. For a while when I first started working digitally I just assumed that the computer knew what 50% grey was and that I simply must be mistaken when I felt that it seemed too light. When I couldn't accept the greyscale jump from 90% black to 100% black any longer, I had to figure out what the problem was. After finding no difference when I changed my gamma settings I figured that it must be photoshop. So here's a comparison with Luminosity and the default greyscale in photoshop. You start to see major differences almost immediately. 50% Luminosity = 62% Black on my photoshop.