What's "regimented" about allowing use of RUS or some other book? Granted, if someone is refusing to sing anything not in RUS, or demanding that the RUS chords and words be used, that's kind of limiting. In the groups I've done, we go around the circle and people can do whatever they choose. The only rule we have is that, if it's not available in a book that's handy, you've got to be able to sing it yourself.