In “How to Practice,” Ann Patchett writes about what she learned when helping a childhood friend clean out her late father’s apartment, and how it reminded her of writing. In short, she says you cannot do two things at once:
I made the decision to wait until we’d finished with the entire house before trying to find a place for the things we were getting rid of. This was a lesson I’d picked up from my work: writing must be separate from editing, and if you try to do both at the same time nothing will get done.
Compare this to filmmakers, some of whom (Spielberg, Soderbergh) are known for editing as they go to save on unnecessary filming. To some degree, writers must do the same, since if you put down everything, you will never finish. Still, Patchett has a point. When you are writing, write. Let it pour out, and worry about editing later.
Within reason, of course.