Being a Nemeth fan, I look for an excuse to justify transcribing according to Nemeth Code. There is no guideline that I know of. If you don't believe the entire book requires Nemeth Code, you could use the Code for those volumes where it is necessary, transcribing the entire volume accordingly. Remember to include the note on the TN page stating the use of Nemeth Code in the transcription of the applicable volumes. As I said, I would probably choose to do the entire book in Nemeth Code.