The climate and the aspect of your garden along with the views you have are the major factors to consider when deciding where to situate your decking. A north facing site will probably be the coolest location. South or west facing sites may be too warm in the middle of the summer, unless you include an overhead screen, or build the decking around an existing shade tree.
Consideration has to be given to prevailing winds when locating your decking as you don’t want it to be too exposed, likewise, careful placement can minimize traffic noise, eliminate unwanted views, or provide additional privacy. If you plan to include a hot tub or swimming pool in your plans, obviously consideration has to be given to privacy for you and your guests.