"The film should be exposed by the light you expect to use when exposing actual photographs. The step tablet as developed on film is what the paper will see and so should be read by blue light for graded paper."

'Just out of interest' how do you think the step wedge densities (rather than the step wedge image on film) should be read? By the mode that most closely represents the light used to expose the film (eg visual), or by the light that will be used to measure the film negative densities (eg blue or UV)?