Is it different to add seasoning at the beginning, in the middle or at the end of cooking?
Yes it does make a difference when you add the seasoning. The taste varies depending on when the seasoning ingredients are added. As for as stir fries or any dry fries are concerned, it doesn't make much difference. But in water based dishes like sauces, curries, gravies, and soups it could impact a great deal. The water absorbs the spices added , making its flavor incorporate completely. Also the raw smell of the spices vanish to a great extent if they are added at the beginning.  Also in case of pepper in gravies and other dishes, the sooner it is added, the hotter the dish becomes after a considerable amount of time. It is due to the time taken for the spice to infuse the flavor/taste completely through the dish.
Dishes based with vegetables, the sooner you add salt, better the vegetables absorb . They could be bland if salt and spices are added last.
In India, in most of the curries masala (ground mix of spices, like cloves, cardamom, cinnamon, fennel ) is sprinkled at last to give the dish an extra flavor, though it is also added in the beginning during preparation.

Hence taste could vary depending upon the time you season the dish. There may be exceptions. But that is the general theory for most of the dishes.