To be clear, so long as the chlorine lasts there is no need for the algaecide. The purpose of the Polyquat 60 algaecide is to continue to inhibit algae growth even if the chlorine runs out or gets too low. If you close the pool very cold and open it before it warms up, then you should be able to keep chlorine in it the entire time, depending on how long it is closed. If you've never done a closing like this before, you can use the Polyquat 60 and then measure the chlorine when you close (after dosing and mixing) and when you open to see what kind of loss you can expect and if the chlorine lasts. If it does, then you don't need to use the algaecide in the future. If it doesn't, then you do.