It really depends what your doing! It is easier to shift your body weight on climbs if your standing up which does make you go faster, also on technical sections or steep downhills it is a lot easier and safer. If you are doing jumps then you need to be standing up and for downhill mtbing i would recommend you stand up all the time unless there is an easy flatish section! But really it depends on what type of riding your doing and you need to find out and experement for yourself!