Unfortunately Arnold, increasingly, there are less and less "rules" that "have" to be adhered to. It really comes down to, well, ultimately, who decides what the output is? Is it you? If so, then you decide what the aspect ratio will be. Is it a client? Does the client care? Does the client know enough to care? Depending on those answers, then it may STILL be you that gets to decide. Is the product going to Theaters? Broadcast TV? Web only? iPads? Android Phones? Art installation? Projected onto the side of a building?
Because you're right: to get the best composition, you need to be aware of all deliverable options before you start shooting. However, everyone is different, and will have different main avenues of delivery, which all have their own needs.
If you're making your own demo reel to show what you can do, then I would say YOU get to decide the aspect ratio.