Is it correct to have either high or low postures in the set movements of Taiji?

The art of Taiji does not distinguish high and low postures, but is rather based on the idea of four "balances" or equilibriums:

balance in the magnitude of the posture or movement such as both sides of the body must have "balanced" amount of spatial displacement when moving;
accuracy or precision achieved simultaneously by all parts of the body;
bodily balance when moving or turning;
steadiness particularly when moving.

External and internal balance or harmony must be cultivated where there is no slanting of the central axis of the body. When hind force is invoked, the hind knee being bent will move up or straighten slightly though the height of the body remains unchanged. This is so as consciousness (or intend) and Qi would "close" centrally instead of coming up while the bent knee is used to adjust accordingly. Consciousness is used to lead the muscles in relaxing. Joints, muscles and ligaments must then be loosened, relaxed and "thrown" open but still linked. The body is then erect and comfortable. Consciousness is also used to "move" Taiji principles to parts of the body. Having achieved "four balances and eight steadinesses," the question of high and low postures is then answered individually.