A pencil drawing can be greatly enhanced by matting it before hanging, but a few considerations have to be made before choosing the color matting to use. Good matting can make a mediocre drawing look great, while bad matting can make a good drawing look poor.
Consider the subject matter of the drawing. The main goal when matting any piece of art is to create contrast between the art and the mat. If your pencil drawing is dominated by darker tones, your mat should be lighter. Conversely, light pencils could benefit from darker matting, possibly even black. Drawings using very soft pencils almost look black, and may benefit from a completely white mat.
Consider the size of the drawing. The standard procedure is that the smaller the drawing, the larger the mat in relation. Because of this, really small drawings tend to have quite a bit of matting around them. Make sure the color you pick for the mat is in high contrast to the drawing so the drawing doesn't get lost in all the matting.
Think about where you'll be hanging the drawing. One of the main reasons to matte a piece of art is to separate it aesthetically from the wall it's hanging on, so the worst thing would be to choose matting the same color as the wall. A wall that is completely white can almost always benefit from completely black mat.
You can even try using double matting if the drawing calls for it.
Make sure you cut the mat carefully. Crooked or jagged edges can make your matting look horrible, no matter what color you choose.