I was leading a group ride last night. We had 10 riders and twice had riders that were climbing yield to us. The trail was very narrow and because of the size of the group, the climbing riders chose to yield. We slowed down and waited until they moved off the trail.
Most experienced mtn bikers can assess situations like this and make judgments depending on the individual situation. I never assume that a climber is going to yield to me, even in a group ride, but they often do and of course when this happens, we always thank the climbing riders for letting us go.