I like to use gradients to normalize tones.... I think it enhances the way the illustration looks and its a very quick way to smooth out otherwise kinda blotchy areas of the painting. Its also a real nice way to quickly lay in lighting and begin to figure out light sources. Of course gradients are kind of blunt, so you have to make them conform to surfaces by either simply "erasing out" the areas you don't want or you use a layer mask. Layer masks are nice because you can continue to manipulate that layer as needed.
I found a great article by the master Paul Felix who gives some great instruction about the use of tones.... I HIGHLY recommend his entire article:
Here is a progress shot.... trying to stay uh, un-constipated