With the idol system its complicated. There are pros and cons.
Keeping in mind that being an idol is still work, unless you really get along, I dont think they should all dorm.
When you get off of work, there are bound to be coworkers you dont get along with and then youre put in a situation where you have to see them almost 24/7.
However, it is convenient when everyone has the same schedule and managers are not rounding up several people from different neighborhoods.