Tish Davidson said:
This works for fiction, but it can be a disaster for non-fiction that requires absolute accuracy or when you are writing to a specified word length. I'm an edit as you go person and it works for me, although I think I'm probably in the minority.
Wrinting without editing as you go can gve some writers problems, but I find hitting a specified length is much easier when I don't edit as I go, and for me, absolute accuracy is why you proofread.
I think it's really a matter of style, practice, and training. Early on, I had a gig writing a short story a month that had to come in at 1,500 words, an article a month that had to come in at 1,800 words and a column that had to come in at 750 words. This on top of my regular writing.
I also worked in journalism where being able to write very fast, absolutely accurate, and at length is not only good, but a requirement for fast-breaking stories. This probably helped to no end.
It didn't take long before I learned to hit any word length on the first try, fiction or nonfiction.