This is poor advice that could be potentially damaging. Instead of manually building and installing from upstream source code you should bump the package version locally to ensure your system isn't littered with orphaned files later on. You'll get the benefit of portage's sandbox and other package management features too. If Gentoo's ebuild had Gentoo-specific patches, you'll get these compiled into the latest version as well. Sometimes these patches are very important.
A better approach to follow is (note this is only a rough guide from memory):
I agree that it's a bad way to install new software but I was merely suggesting a way to upgrade nginx that was installed this way in the first place, I assumed the OP meant that by asking for a "stable way to update nginx installations from source.". The fact that I did it on Gentoo is irrelevant here.
I love portage, I use it whenever I can. In this case, I think I've did it like that because something was messed up with Passenger support in the port. It's the only package installed from source (bypassing portage) on my system and it's not an orphan in a way that /opt was dedicated purely for such scenarions. I can see all such packages by listing /opt assuming I keep the install prefix convention.
In any case, thanks for pointing that this is a wrong way to install packages, someone might benefit from this indeed.
Indeed this is what I was looking for (however I will be updating the software on CentOS) sprinkled with a few snippets of wisdom from the above posters.
A better approach to follow is (note this is only a rough guide from memory):