[AnimalMuppet]

Use formal language when the audience demands it, and never otherwise. Why? Because your audience is human beings, not machines. Write for humans.

Even in reports and documentation, your audience will thank you if you sound like a human. Now, documentation has to be precise, and therefore has to define and use terms precisely. But documentation also contains description, and that description should be written informally when possible.

Edit: Ignore this. ucarion said it better.