Mudra is called Shuo Yin (Hand-symbol 手印) in Chinese. And it has been popularly used in Eastern practices, including meditation, religion, simple good health, martial arts as well as traditional dance. The fundamental reason of using Mudra is its ability to generate chi in the body, making a practitioner's body "more in shape" as well as allow chi to be directed to different part of one's body according to different objectives that one may like to pursue. What then is the particular objective of using Mudra in meditation, and Taoist meditation in particular?
In standard Taoist meditative stance, one doesn't seem to see Mudra being practiced, and hence gave people a false impression that there is no Mudra in Taoist meditation. As the photo below shows, the Mudra used is actually a concealed one: covered by the outer hand is the inner hand's thumb touching its middle finger. The primary objective is to generate chi together with directing chi to move along the center line, most essential for cosmic circulations.
This concealed Mudra is however only the form that is best suited to direct one's chi up the spinal cord and down the front (i.e. doing microcosmic circulation 小周天). To achieve a solid foundation for eventual deep chi mediation, one must also be trained to have chi well balanced within one's torso (or metaphorically called cauldron 鼎) together with strength and power. To achieve this objective, classic Taoist teaching called this "opening the eight psychic channels" (please refer to my other posts on the eight channels for more exposition). For the uninitiated, it may just mean opening "small ducts", but the actual requirements will be "filling one's cauldron with chi by FIRST [which of course is essential] opening one's eight channels". What then is the relevancy to Mudra? To answer this question, we have to ask the martial artists.
A key objective of a martial artist is to build up a strong body ready for combat. And for those who practice internal martial art, they will use chi to do the strengthening. As a result, in addition to zhan zhuang stance for health purpose, there are many stance called combat stance in zhan zhuang that aim at thus strengthening one's body, and in these combat stance, the hand positions, or Mudras, are many.
Therefore, I would suggest intermediate and advanced meditators to freely make use of different kinds of Mudra (taken over from other disciplines when required). The objective is to build up one's total body with balanced and strong chi. Which to choose? That is actually rather easy for seasoned practitioners. Without any limiting thoughts, a practitioner can feel by himself which Mudra works best for which part of his body. Like a fish knowing exactly where the water is warmer and where it is colder. If you can't feel it, you are not yet there: practice more!