Let's remember what the purpose of a topic sentence is. A topic sentence states the main idea of a paragraph. It is an organizational signpost so your reader can clearly follow and understand what you will be discussing in the rest of your paragraph. If you wait and put the topic sentence at the end of the paragraph, how will your reader know what the topic of the paragraph is? The reader needs it at the beginning to insure that they can follow the structure of a clear, focused writing. That doesn't mean that you can't have a transitional sentence or important detail before the topic sentence, but to make sure you are communicating effectively to your reader, it should be as close to the beginning of the paragraph as possible.