In the semi large projects where I usually work (more than 2 million lines of code) I would ban private class functions if I could. The reason being that a private class function is private but yet it’s visible in the header file. This means if I change the signature (or the comment) in anyway I’m rewarded sometimes with a full recompile which costs several minutes (or hours depending on the project).
Just say no to that and hide what’s private in the cpp file.
If I would start fresh on a large c++ project I would enforce PIMPL Idiom: http://c2.com/cgi/wiki?PimplIdiom to move even more private details into the cpp file.