@evan
I can’t answer this question without more clarification.
What are you counting as physically damaging a book?
Getting any creases in the spine or pages? (So, if you read a book you damage it, in which case I say always)
Making markings on the book pages?
- writing name in front cover (often, not library books or loans, always for personally owned books)
- underlining, highlighting, notes in margins (if you own the book, go ahead, there are also degrees to this and didn’t reasons for it—puzzle books, journals etc are books that are made to be written in, for prose, if it helps the reader interact with the content in a way that benefits them and they own it, great)
Ripping out pages/physically dismantling the book (rarely, but if you own the book and have a reason to do so, ok)
Rendering the book unreadable by gluing the pages together, cutting holes for storing valuables, or burning the book (rarely, but there are acceptable reasons)