Editor's note: There's a section that talks about asking the preference of the people you write for on using contractions. For the record, we neither find it informal nor unprofessional to use contractions. In fact, I would go as far as to say that contractions can be effective in making writing sound more conversational, which is always a good thing in my book.
At the end, the video also references consulting a style guide. Here's our style guide.