I do not agree with all said above. It encourages us to produce second-rate work.
Any great writer, artist, musician...and so on, strives for perfection.
I agree that at step one, I concentrate on letting the story flow so that I get it on the page as fast as possible. But afterwards, of course I'll go back to proof-read and try to improve what I wrote.
I only want to 'own' the best I can do, the rest I throw into the fire.