Regarding S/L out of a C-182 only: I have never shut the door between students. A gear check is completed and the student is hooked up prior to movement into final position.
S/L handling has never been made difficult for me because of this. I can understand how having a PC in the picture might change things, however.
I also think closing the door for student comfort is a valid point, but would probably only do so if the student were in serious need of calming.