Harmony or balance in life comes from having the wisdom to know the difference. This wisdom cannot be given to you or taught to you. It will come through your own experience.
Listen to your body, go to your first edge and when, and if, the body opens and invites you in deeper, then accept the invitation and go to the next edge. Once at this new edge, again pause and wait for the next opening.
You do not need to go any further if you are already feeling a significant stretch, compression, or twist in the body. Going further is a sign of ego; it is not doing yoga. Staying where you are is embracing yin.
However, this is not an excuse to stay back and not go deep into the posture. When we work at our edges we come to the point of significant resistance.
This will entail some discomfort. In this way, Yin Yoga is not meant to be comfortable; Yin Yoga will take you well outside your comfort zone. Much of the benefit of the practice will come from staying in this zone of discomfort, despite the mind’s urgent please to leave, to move, to do anything but stay. This too is part of the practice.