I have the same philosophy. I also won't let them start over unless the first picture is finished. I state this rule in the begining of the year. "You need to finish the first picture and then you can get another piece of paper and start again". Nine times out of ten when finished they like the first piece. Occasionally I will get a few that rush so they can start over. I recently had one student like, then dislike, then like, then dislike, and finally when it was done she ended up liking her work. Another saying of mine is "keep going it's gonna change!" I tend to design lessons that often change the original idea to something completley different.
Just to help them understand things don't always take the path you thought they would. Of course some lessons are so involved this may not be possible but I'm elementary so it generally works.