Well, using skin texture as an example, if the person looks like they are a field laborerer who's 100 years old, with the bump set high, it's probably too high. You want the skin to look real not like scar tissue. A bump map can make a lot difference in a man's texture especially, and used delicately, in a woman's as well. And, of course, it's crucial in Dragon texture. As for experimenting with, like, wall texture-- it's pretty simple. If inverting the bump map makes the grout look higher than the brick then don't invert it. I get mixed up on which is high and which is low-- I think white is high on a bump. [I mix up my right and left too]