When you need to make large changes to your iTunes library, such as changing the genre for 100's of files, or cleaning up artist names, there are several ways to go about it.
One of my favorites is to hit Apple-B and reveal the browser. This will put a three-paned window on the top of the list view.
Now you can select the genre or artist that you'd like to change at the top, type Apple-I and make changes in bulk. When you are done, it is also quite easy to visually see your changes in the Browser.
I think this is much easier than sorting the normal window by genre or artist, selecting items to be changed, then pressing Apple-I.