You may want to just use it on every page.

One good argument for using a self-referencing rel=canonical on every page is to combat scrapers. If they grab the entire code, including the rel=canonical, they are essentially telling the bots that your page is the original, and they'll be much less likely to outrank you for your own content.

Larger sites tend to generate the rel=canonicals automatically for every page, and give you the option to customize it where necessary.