CVS can at times seem like a bewildering collection of improvised standards. There's RCS format, various output formats (history, annotate, log, update, and so on), several repository administrative file formats, working copy administrative file formats, the client/server protocol, the lockfile protocol.... (Are you numb yet? I could keep going, you know.)
Fortunately, these standards remain fairly consistent from release to release – so if you're trying to write a tool to work with CVS, you at least don't have to worry about hitting a moving target. For every internal standard, there are usually a few people on the email@example.com mailing list who know it extremely well (several of them helped me out during the writing of this book). There is also the documentation that comes with the CVS distribution (especially doc/cvs.texinfo, doc/cvsclient.texi, and doc/RCSFILES). Finally, there is the CVS source code itself, the last word on any question of implementation or behavior.
With all of this at your disposal, there's no reason to hesitate. If you can think of some utility that would make your life with CVS easier, go ahead and write it – chances are other people have been wanting it, too. Unlike a change to CVS itself, a small, standalone external utility can get wide distribution very quickly, resulting in quicker feedback for its author and faster bug fixes for all of the users.