I think that it is good to show an excellent example of what the students should strive for.
I also think it is good to show how to do it by example.
After that, what the students do - unless they ask - is up to them.
I'm not in favor of making a student redo a seam over and over - especially for a beginning beginner's class- but I'm not in favor of the instructor saying "that's wonderful - when there is lots of room for improvement.
One can say things like "Good try . . . . " "Next time, try to . . . . "
It is also good for an instructor to show how a boo-boo can be salvaged. That may be one of the most important things to learn!