This is not a particularly common habit, and the management of it to some extent depends on the nature of the behavior. Sometimes children experience dryness in their fingers and remedy this via the behavior. Other times it represents a compulsive habit that soothes some intrusive or obsessive thought. Ask you daughter about it and see if you can figure out what drives the behavior. In a practical vein, be sure her fingers and hands are not overly dry. The greatest risk in the behavior, of course, is the enhanced chance of introducing viruses and bacteria.