It all depends on what you want to show in your art. If you want to make color stand out, or rather make the painting all about color, it can be effective to reduce the value contrast. Because value contrast is much more noticeable than color contrast, when you take away strong value contrast, the eye will see the next most noticeable thing, which in this case, is color. I like having strong darks and lights in my art. However, if you look at any part of one of my paintings where there is not a strong value contrast, you will see an area that is subtler, where I am playing the colors against each other for more interest.