I'm not sure a teacher is a must have. What I believe is a must have is access to others who can help you see when you're headed down a blind alley, suggest practices that might benefit you at certain times or just be there to talk and compare notes with. Having access to those who have been through the territory is a major boon to a practice, IMHO. I guess I'm suggesting sangha is more important, be it live or be it virtual.
I'd check the "strongly agree" box next to this one. Sangha is where it's at.