First of all, if you take the CNAME first, you're safe. GitHub Pages will raise an error to anyone who would try to create the same CNAME file. This if a bit frustrating with forking such repository.
On the second hand GH Pages recently added an option to set DNS to a specific account, using CNAME
DNS record instead of A
like this:
www.example.com CNAME username.github.io
Checkout the page you mentioned for more info: https://help.github.com/articles/setting-up-a-custom-domain-with-pages