I just finished a new self portrait drawing. As I was drawing it, I realized that the values have to be accurate in order for the drawing to be successful. This applies to all drawing as well as painting. Personally, I think I used to be afraid to make the darks look dark enough, but now I've realized that all the power of art is in accurate value.
A really helpful tip to check your dark values is to squint at your subject and then compare it to your drawing. I don't mean to blur your vision, but to squint your eyes down in order to assess value more accurately. The reason squinting works is because it gives you a more accurate sense of the lighting and thus, the value. It also takes away unnecessary detail and boils the world down to a format that will look good on paper or canvas. Don't be afraid of making the darks darker if they look dark when you squint! And always compare your drawing with your subject!