…that is just one of the questions that all writers have to answer for themselves.
The issue is also one that can’t be answered by anyone but the writer. Every writer will answer the question to a different degree.
I’m a very structure oriented person; a plot is important to me. Even in life I prefer to have some sort of outline of what is expected of me and what I expect, so it comes as no surprise to me that my writing would use that as well.
I typically have a multi-stage process for my plots that starts as a general overview of what I want to accomplish in each novel and ends as a chapter-by-chapter outline with blurbs about what is going to happen at each step.
Even in the most detailed approach, however, there has to be room for your characters to breath, change things about themselves and the path that they are on and the path that your story is on.
I liken my writing approach to those hurricane predictions you see on TV every year. Nearest to the storm, the path is small; as it moves away the possibilities broaden but there is still the wider path.
The importance of maintaining some bit of flexibility is important because you don’t want to tie your characters down and force them to do things they wouldn’t.
In my writing, this has resulted in some strange story threads that, at first, confused me. Now, however, looking back I realize that the flexibility that I allowed my characters was essential to moving the story along.