(There have been a few commission I’ve ripped up and started over, but those are one-offs, so there is no “next time”.)

Funny, While attending the NYCC a number of years ago Marvel was hosting a panel detailing the process of creating a comic where they broke down the process from  script to printed comic. I remember posing the following question: 

‘How do one submit artwork that you KNOW  if you had a few additional days, they could make significantly better?’ 

Their response was two-fold: 
ONE: an artist must always realize that you are just ONE PART of the comic creative process (artist/Inker/Colorist/letterer/editor/printer) if any one in that chain ‘pushes their time out’ it affects the whole process.  
TWO: An artist (writer/illustrator/inker/etc.) can only improve in their craft if they learn to work within guidelines/deadlines. If you typically work on a piece until its ‘perfect’ you will never improve upon your craft. 

(Essentially, 'soldier on and “get it right the next time…') 

Makes me think to back to an artist I met at a convetion once who was offering ‘6-MIN COMMISSION for $60’. Once paid, he would literally start his timer and at the six minute mark, he was DONE!

Also a day or two ago, while hosting a video tutorial, I asked Adam Hughes the very same question and he gave the very same answer.