You don't notice it unless you are running Live Headers or similar. Of course if you code absolutely all redirects yourself, you can do them as a package. But if you are using apache's built-ins and/or stuff that your host optionally does on your behalf, then with/without www and directory-slash is each its own redirect.
That's why I wondered if g### treats all redirects alike. Now, I'm not big enough to get requests for nonexistent files-- or maybe, ahem, there are enough naturally occurring 404s that they can't have any doubt. But I do notice that whenever gwt checks for its validation file, there are two requests, one redirected and one not. I have to assume that it's asking for the with-and-without-www versions, and noting the fact that one of the two gets a redirect. (Being a robot, it can choose not to follow a redirect. Doesn't seem quite fair does it?)