One thing to keep in mind is that the Harry Potter books were written by a British writer. I'm not sure why, but in my experience I found that Europeans tend to use more exclamation points than Americans do. Regardless of whether English is their first, second, or third language, they like exclamation points. Used to drive me nuts editing the corporate newsletter or their trade magazine articles.
First, I had to take out all of the exclamation points. They really had no place in a technical article on the merits of shotblasting vs shotpeening, or process development case studies.
I've noticed that Canadians, and a friend of mine who works for a Canadian company (but is from the US) also use them more generously than Americans. It's used for emphasis, and how I can always tell when my exclamatory friends are happy.
To me, an exclamation point is the standard English equivalent of ALLCAPS, and indicates a raised voice/shouting. I use them rarely.
So, I'd look at your target market. How do they use exclamation points? Do they use them when they are happy, or when they are yelling?
Focus