The word you use for a topic will be the index page of your site. If wcb goes and gets 300 keywords to build sub index pages on there is a good chance you will get ads on the long tail keywords. Many people don't put ads on the index pages so they are not seen if they get hand reviewed.

 WCB is a bulk building tool. If you get a site and are not happy with the number of ads you can always change the topic. What I got from the video on topics you don't need to worry about topic research. Just keep building.