If your pool is likely to turn into an icecube over the winter, then the most important step in winterizing is lowering the water level below the return, blowing out the lines, and draining the pump and filter. If you close when the water temp is consistently below 60 degrees and open when it is consistently above 60 degrees then you shouldn't have any issues with algae. If you do use an algaecide look for a polyquat version and avoid anything with metals in it.