When moving, or otherwise transporting large numbers of books, do you care how they're packed?
No. What matters is they get there.
Yes. Book order (as determined by how they were arranged on shelves) is the most important book-packing criterion. Spare space can be filled with bubble wrap or styrofoam.
Yes. Book size is the most important book-packing criterion. Fitting them into boxes so they're safely packed is like a puzzle.
Yes, for another reason which will be explained in a comment.
I packed by size. I can always re-order on arrival. I care most that the jigsaw puzzle of books are securely packed and won't be harmed. Thus it was thought-provoking to read jandersoncoats's reaction to her movers packing her books out of order. I presume fjm and chilperic must have packed their books in order too. They had friends emptying boxes onto shelves as their shelves were installed.