I'd say that for beginners, keep MDM on. As for bad habits, there are far too many fundamentals to learn first, IMHO, before concerning oneself over how to handle an oversteer. Learn the line. Learn how the shifting weight of the car feels as you run the course. Let the DSC nannying teach you how to move more smoothly and quickly through corners first.
Then step up to turning DSC off, when you have a bit of a feel. I'd hate to be the one to say turn DSC off and have a novice spin right in front of another novice.
Remember those other novices out there. The ones that have learned how to handle their cars well have moved up to a more advanced run group.
To me, DSC-off is a more intermediate learning step. I'm sure there are Crash Jacksons who will disagree.