I usually lighten any flat black that is supposed to be a painted surface or a black material. I leave unadulterated flat black for representing cavities that I do not want to open into real cavities.
For gloss blacks, I do use it straight.
The amount to lighten the flat black is not really critical- any lightening helps. Surfaces on top of model chalk from sunshine, so these should have more lightening than blacks on the side of a vehicle. And blacks down low- tires, chassis, etc. can get dusty easily. I tend to lighten these with a tan rather than a gray.