Chris:
Been a while since you have posted. Go ahead and close the pool. There is no hard fast rule on allow water temperatures to get that low, but a lot of people wait because they incurred a green swap in the spring/summer. The longer you wait, of course the better you are. In all honestly, the real key is to open up early. If your levels are high, just bring back up to SLAM and close the pool. Do not add algaecide. If you like the idea of algaecide, then you may need to wait until levels come down or just add the algaecide, but my experience has been that the PolyQuat will eat away at the chlorine. Some members have posted that the algaecide has not affected chlorine levels.
I do not really SLAM before closing, but bring my levels up. Even if some chlorine is consumed, no big deal. I use PolyQuat every year. Close my pool either the last week of September, and no later than first week in November. Always open by last week of April, one month ahead of the holiday. Never any issues and clean up is within a few hours. Although, I have a solid tarp. If you have a mesh cover, you could take a tarp and cover the pool until the weather becomes real cold. Then in the spring, you could place back on and then take off. We did this with a friend and he had much success. His cover was my old cover and he cut some parts to allow water to drain from the solid tarp through the mesh cover and into the pool. Hope this helps!