Picasso was touring an elementary school and viewing the artwork the children had on display. He was amazed at how good it all was.
He asked the teacher...
"What are you teaching these children such that they are all fantastic artists?"
To which, the teacher replied, "I simply know when to take things away from them."
At some point, experience teaches when things are approaching a "singularity". That stage where more changes will simply be more changes requiring more changes all without considerable improvement, or even possibly causing degradation. It is difficult to state when that singularity occurs. However, when I find myself laboring over minute things such as "that square serif or that round serif" rather than simply serif vs sans serif. Or, "10% cyan v 12% cyan"... then that's a good indicator that I'm becoming too vested and making too many minute decision at the moment.
While some of these decisions can absolutely help a project, if I find myself searching for things which I can change in minute ways, I'm too close to a project.
I leave the project for the time being. Walk away... for a day or two.. then return with fresh eyes.
Fresh eyes are a greatly undervalued aspect in design. It's far too easy to get so involved in something that you miss the larger picture. With fresh eyes you are forced, naturally, to view the work in a different perspective from a different "path". With fresh eyes you may find that all those minor things you were up in arms about are all unimportant. A new perspective may also show you things you were missing previously.
It is also important to keep the audience in mind. While some things, like font choices will alter perception, as you posted, the general public typically won't be swayed by two typefaces which are very similar. For example, Helvetica and Arial. Unlike helvetica, I find Arial down right ugly. However, 95% of the public doesn't see any difference between the two. So broad strokes often matter far, far more than detailed strokes. So large choices, i.e. serif v sans serif , are far more important in general than which sans serif*. I'm generalizing, but I'm sure you get the gist. I mean Comic Sans is drastically different than Helvetica
With web projects, there's also the "refine later" aspect. There is both a benefit and detriment to how web projects are often deployed. A detriment because they can be ever-changing, never-ending, always present, tasks. Especially for oneself. However, they can be beneficial because one can make changes quickly and test if minor changes are a factor. One can run a site with Helvetica for a month, then switch to Arial to see if it makes any difference. Educated changes are always better than "in my head" changes.
Ultimately there's some discipline which is gained through experience where one learns to stop working because you are merely spinning your wheels at the moment. And even something like a 1-2 hour break from a project will possibly result in a better perspective on what may need alteration, if anything.