Two months ago we went to Blockbuster for a couple of movies, and at the children and young audience movie aisle, I was shocked (duh!) to see that ALL classics of children literature have been made into a movie or more. That's also the case with many adult books.
A few years ago my girls watched Charlotte's Webb the movie, they also have a cartoon version of it. So this summer, while we read Charlotte's Webb together, they constantly remembered the movie. Somehow I thought it would have been much better to have watched the movie or movies after. But that is not always the case. You may want to watch some Shakespeare before, to prepare for it, or so I've heard... we are not there yet.
It will be difficult to say what the best way to go about with movies and books is. Their love for the Wizard of Oz movie didn't hinder their desire to listen to the book, and they had seen Mary Poppins before we listen to the book in audio. I don't think they realized the Mary Poppins in the book is different than Julie Andrews, but I love Julie Andrews' Mary Poppins, what can I say?
Sometimes I've seen the movie of a book I've read afterward, although when I was in college it used to be treachery to watch the movie before reading the book. Those were the times when I read not just for growing or for the how pleasant it is, but to be able to say I had read the book, and sometimes the popular books were 'bricks' as we call them, because they kill you with boredom and you can't eat them, or garbage, but fashionable garbage.
Probably, the most damage done to a book it's the Winnie the Pooh, not the movies, that I find well done, but all the baby paraphernalia as we discussed at a Winnie Party my friend Kim threw for our children who were and are reading his books, because it reduces Winnie to a baby shower, clothing and baby articles corner.
So movie or book first, does it matter? There is no rule of thumb, but for young children, I still think it may be best to read most of the classics first or they may feel inclined to believe there is nothing in the book for them. On the other hand, we've seen Heidi cartoons I used to watch when I was little, and I don't think it will spoil the book. As for Harry Potter, if you read/watch the books and movies, that I don't think it matters what you do first. It's more of a caution for some books that you may find can be spoiled in your home by viewing the cartoon or movie first, as well as the awareness that many movies may be based on books and you (at least ignorant of many American classics me) may not even know it. Once you know both options exist, you decide what it's best for your children.