Book figures are a good guide. However in certain latitudes if you climb at the book figures you wont make it into the colder air. Eg., if you are in temperatures of say ISA + 18 until FL330 or FL350 the book values may not get you high enough to get into the colder air. Therefore a lower ias/ mac no would be more beneficial, to achieve a reasonable R of C. At FL400 you then maybe at ISA. Yes you will loose a few minutes, so it is a trade off.
This does not only apply to LJ's