I have seen two recomendations on this. Some say no bedding, some say only a tiny bit of bedding.
I use no bedding, and have no problems with my larvae initiating, maintaining or completing the pupation process.
Within a couple of days of being left alone, they typically curl up and go dormant. Several days later, they shed into a pupa, and a couple of weeks after that, shed into a beetle. No bedding/food is needed for this stretch of dormancy.
If you are getting into breeding, I find the selection of worms for pupation to be of far greater importance than the presence of any bedding. Only pupate the largest of your superworms. Smaller ones simply don't have the energy stores to make it through their metamorphosis.