I always asked all the questions I had, even if I felt that they were silly questions. This is primarily because I felt really comfortable with my supervisor and I felt she was so helpful with my small questions.
I think that students need to be made aware that that is the supervisors job, and that if this was a regular MPH that's done in attendance (not via distance ed), that it would be even more collaboration and talking with the supervisors - because that's what is normal during a master's. I think setting up Skype chats or collaborate sessions for "longer" chats is a good idea for students to do throughout the year (even just two or three sessions), but to reassure them that small questions can be sent via simple email or discussion boards on learn without feeling that they are over burdening the supervisor.