When I was in college, back before every mother's child was supposed to be gifted across the board, I knew at least two people who let some of their course work go in order to focus the bulk of their time and energy on their major areas. They didn't fail anything, they just didn't worry about taking a B or B- for a class so long as it wasn't in engineering or nursing. They were not being shiftless and lazy, they were strategizing. They went on to successful careers, by the way.
According to Daniel J. Levitin in The Organized Mind, they were satisficing. Levitin says Herbert Simon came up with the term "to describe not getting the very best option but one that was good enough. For things that don't matter critically, we make a choice that satisfies us and is deemed sufficient."
How does that apply to writers? We have to manage our entire lives in order to make time to write. But we can't just blow off things, leave them totally undone. Remember the environmental disorder study last year that found that a disordered environment leads to self-regulatory failure? We definitely can't just ignore taking care of our homes, for instance, to make more time for writing because the environment we make for ourselves could lead to problems with staying on task. Instead, we have to find a level for what doesn't matter critically that is satisficing, good enough.
It's even true for professional work. Our work situation is always changing. When we have a contractual deadline, we have to focus on writing, but we can't just forget about marketing and other types of writing. They aren't critically important at that point, though. We satisfice. When a new book comes out, marketing does become critically important. We satisfice with the writing then.
We move among all the things we have to do, focusing our effort on some things and spending only as much time as we have to on others.