Two things I found:
Along with $Id$, which shows the last updated date as well as who made the mod, you can use $Log$, which will insert the comment you used when you checked it in.
Also - while playing with the feature, I checked a file out and then committed it back in without changing anything. I noticed that the header info did NOT change. So, at first I thought I’d be required to add $x$ to my header each time. However, it appears as if CVS was smart enough to notice that I didn’t change anything. When I tested again and actually changed the file, it worked great.