While working on already existing code I often find font-family declarations spread throughout hundreds or sometimes thousands of lines of code. This is somewhat frustrating when my task is to change the font families, because I have to search through the CSS to find all the font-family declarations and then build a mental image just to figure out the current system behind the font family usage.

With font-family, you should have a clear plan for your website, and typically use no more than two or three font-families. If not, you will quickly end up with chaos and inconsistency, two qualities you don't want to be associated with your code. I prefer to gather all font-family declarations in the top of my CSS like this:

One weird problem that I have observed a few times is that a website has been shown with a narrow font on one computer. While you can say "who cares" as long as it is only one computer, you only test on a few machines anyway so one might be a significant percentage and indicate that you should do something about it.