In the Iyengar tradition, backbends are considered energizing and are thus placed earlier in a practice sequence. Is it necessary in Yin to do likewise, or can sphinx/seal be placed in the middle or toward the end of a sequence?
There are many ways to practice yoga: no one way is the "correct" - it really comes down to your intention. The Iyengar tradition is a wonderful one and backbends can indeed be energizing. Other traditions use backbends as counterposes or heart opening and position these postures at other times in the class.
In Yin Yoga you may place backbends early, to stimulate the Kidney meridian, or in the middle, to balance some earlier forward folds, or towards the end so nourish the spine. There is no "correct" time to do them but there are better times and worse times. I have never seen backbends done at the very last, just before Shavasana, but I have seen them done in the middle of a practice and often will teach this way.
Remember your intention (why are you doing backbends?) and let that guide you in deciding when is the best time to use them.