Perfect positioning or perfect stillness and complete relaxation. We want it all, but I'm haven't achieved that combination yet. Like you, I start out with the correct posture but as I relax my posture does shift. Most of the time I opt for staying relaxed, which I equate with being open, and still. Other times I will reset my posture, and relax and be still again.
When I was starting I used to feel my fingers were all curled up and several times I opened my eyes to find only a small bend in them. I have also peeked to check my sitting posture, and again it was not as bad has I felt it was. These peeks have helped be choose to remain still and relaxed.
However, I still want to achieve and maintain the best postures I can. So sometimes I correct my posture during meditations. I think of it as working on the yang side of the exercise. Eventually the yin and yang portions of the exercise will improve and go from a good balance to a better and eventually the best balance.
As for working on you back muscles, I fine myself correcting my spine (posture) all the time. While typing (just now), but especially when I walking. Just a quick correction of my posture as I hear "Your back is getting longer, longer completely straight. You channels are completely opened." This two second change only takes about three steps to occur and my mental attitude changes along with my physical posture. I'm hoping that these little daily corrections and some very rare periods where I work on posture only will help my move closer to the best pactice of the SFQ exercise and create the best flow and balance in my life.
You are perfection.
Iam2 
