How to choose sites to block

You'll probably want to block some of the AdSense ads from appearing on your site. As well as blocking rubbishy sites, you may want to block tough competitors.
The ability to block sites is especially important for sites that are not purely affiliate-income driven. For example, if you're selling a service or a product you won't want competitors' ads on your site.
You can find such competitors by doing some searches on Google for key phrases that are important on your site and looking at the AdWords ads that appear