I read about Google Disavow tool when they launched it and they were advising the webmasters only to used this tool if they are sure if the website linking to them is a spam or bad. If you are using Disavow Tool for every many links then who knows that your website may actually get a downwards ranking because the link you think as bad may be helping you in getting the rank. So only use Disavow Tool if you are absolutely sure that a particular link is a bad link.