I guess it would depend on your purpose for reading the book. Having said that I tend to PR entire books even if there are sections which I don't think I'll need consciously. I might activate all sections that I think are applicable then why not ask you unconscious what else might be useful in this book for me to know. Maybe nothing..maybe something..Maybe an entirely new train of throught that the author or a teacher has never thought of. This I think is the real potential of PRing. You never know, the none activated stuff may appear in an other subject and you'll know exactly where to go and look, this would save you alot of time!
