I'm still looking at the Four Reliances (or Reliables), which are guides for practice found in several Mahayana texts. I've learned that the original Sanskrit word translated as "reliable" is pratisarana. The root "sarana" means "refuge," and it's the same word translated as "refuge" in the Going for Refuge ceremony.
The first of the four guides usually is translated Rely on the dharma, not the teacher. As I understand this, very basically this Reliance is telling us that the truth of the dharma is the priority, and the role of the teacher is subordinate. Or, the role of the teacher is to reveal the truth of the dharma.
That sounds pretty simple, but I know we get it wrong a lot. This is not always a teacher's fault. A lot of us go through at least part of our lives looking for a spiritual Mommy or Daddy, and when we think we've found one we easily get attached. In my experience, good teachers do not encourage this attachment. But a few do encourage the attachment, and this includes a few with impressive institutional credentials.
I've been told that in Asia it was a common practice for monastics to travel from one monastery to another to receive instruction from different teachers. It occurs to me that this was a healthy practice for the monks and nuns as well as for Buddhism. Constant cross-pollination kept the teachings alive and honest.
However, I want to be clear that I'm not encouraging spiritual tourism. Spiritual tourism is flitting from one retreat or dharma center to another without making a commitment to one path of practice. Truly, this is just skimming the surface. On the other hand, I run into people who seem to think all wisdom comes from the mouth of one particular teacher, and that's attachment.
If you're open to the dharma, you find that the dharma is always teaching. However, the guidance of a teacher is invaluable to help you clarify what you experience and to sort real insight from wishful thinking.
My personal experiences with teachers have all been positive, but maybe some of you have insights from your own experiences to share.