Best practice to use "Canonical" in Smart Crawl Settings on a Category?

Hi there,

I have setup a couple of blog categories like Apps, Books and Courses. There is a main page on my site for each of them as well. http://www.excelstories.com/apps

– How do I use "Canonical" on a category in Smart Crawler settings for best impact. Should I enter my main web page for each category there or just leave it blank?

Cheers

Acke