No, many people get this wrong. Mainline is stable, and http://hg.nginx.org/nginx/ is dev. The labeled stable version is for distros that have strict upgrade policies.

nginx recommends using mainline over stable: "We recommend that in general you deploy the NGINX mainline branch at all times." [1]

[1]: https://www.nginx.com/blog/nginx-1-6-1-7-released/