I don’t think the usual practice is to track individual contributors for every source file (unless that file was copied over from some other project), just to give a brief comment that the file belongs to such-and-such project copyrighted by so-and-so and others, and that it is under license X with a pointer to the license file.
It’s true that a lot of large organizations (GNU, Mozilla, etc.) do this as a matter of course, AFAICT not because it’s necessarily legally essential but more as a precaution in case a file gets copied to some other project, and it’s easier to just make a blanket coding standard than decide whether to include a copyright header on a file-by-file basis.
FWIW, here is a blog post by a lawyer (from the Creative Commons project and other prominent orgs) on the topic: On the Importance of Per-File License Information – Luis Villa: Open Tech and Policy
For my own part, I sometimes do it, but for smaller projects I’m often lazy about it and just use a single LICENSE file.