I hate to say it, but it all depends! Whether to have eyes open, closed or half-closed is a question that often arises in meditation. Some traditions insists the eyes be open (such as in zen meditation) because if the eyes are closed, the student will tend to drift off into dreamland. However, other traditions insist that the eyes should be closed, because open eyes invite distraction through the sights we take in.
Which is better really depends upon you, and your current state of mind. If you find it is easier to stay present with eyes open, then keep your eyes open. Open eyes are a good choice if you feel tired or if you find yourself day-dreaming a lot during your yin yoga practice. However, you may find it is easier to tune into the sensations in your body while you hold the poses if your eyes are closed.
I personally choose to keep my eyes closed, so I can really climb inside the experience that is arising right now. But, if I find my mind wandering too much, then I switch to eyes open. The key is not to allow your mind to use this an excuse to keep switching modes. This should be take as permission to open your eyes for 30 seconds, then close them for 15 seconds, then open them again, then close them again, etc. That is just the mind playing its monkey tricks on you. Choose one mode and stick with it for a while; only if you find it is not working after several minutes, then consciously choose to switch modes.