Here is where I confess I didn't want to learn any rules when I was younger. I just wanted to do things my way. I was frustrated much of the time. Nothing came out the way I wanted it to. Now, I believe in a combination of working intuitively and pre-planning. I think you can pre-plan in a loose manner, just enough to give yourself a structure. A few constraints can be helpful to focus your idea, and that's where the rules come in.
Like most of life, we have to hold two different and sometimes conflicting things in our hands. While I have said in the past that there are no book art police, I was referring to aesthetic choices for content and materials. Aesthetic choices are yours and involve your tastes and preferences. But there are technical police. If a book (or writing or other art form) doesn't hold together, if it falls on the floor, if the cover is unintentionally lumpy, if, if, if anything is unintentionally something else, it's wrong. If your aesthetic choices lead to one of those unintentionally wrong things, then you need to choose again. Yep, there are some things that are wrong. Nobody likes to hear that. Hang on a minute. Here's an example.
The turn-ins on the left are just technically wrong. The corners have been cut too close to the board, and the board shows unintentionally. The intention is to cover the board and its corners. The turn-ins on the right are correct. The board and corners are covered.
Can you fix it if you have done it wrong? Sure, there's the band-aid way: pasting the corners you cut off over the gaps. It's not a completely invisible fix, but it is certainly less noticeable than leaving the naked gaps.
Picasso's statement "…so you can break them like an artist" inspired me to think of a new fix for the problem: paste corners over the gaps that are different colors or patterns. Every time you have a new component, you have an aesthetic choice to make. Here, you could intentionally highlight the mistake, or you could even cut your corners too close deliberately because your content is about pushing through difficulties, making do with what you have, or adapting. Always take the opportunity to use your imagination.
Below, board on the left has technically correct turn-ins,
board on the right has the eclectic fixes.
"…break them like an artist." On the other side of the rules sit the intuition, the inventiveness, the spontaneity, the imagination, the magical art part. Once the technical skills are in your body, you really can work intuitively. A common mistake is to think there are two diametrically opposed ways of working, to pre-plan or to be spontaneous. In fact, and this is why I think Picasso (if it was Picasso) said to "Learn the rules like a pro," was that pros just do. The planning happens subconsciously, it is built-in. But it is only present because you practiced, you mastered the technique and can use it effectively. You may even dream an idea and find that it actually works in waking life.