The best publishers like a certain amount of balance in their work. This trait serves you well if you’re publishing, say, a flyer. Having an equal border on all sides in this case, makes sense. When you publish a book, however, things may be a little different. You will need to take into account the left margin may have to be larger. This is because it will have to have a line break for the eye, but also will have to accommodate the bindings of the page. Depending on the type of binding, the amount of actual margin space will vary. It is worth noting that staple inside stitch bindings take up the least amount of margin room, while perfect bound bindings, and other glue-based methods will require more left margin space. When in doubt the publisher, or POD service can usually give you a guideline.