I just let mine leak through the winter--it often is helpful in locating the leak, because the water level will stabilize when you get to the level of the leak. It also drastically decreased the amount of water I had to pump out all at one time when it came time to drain to replace the liner. However, I'm in the South, and don't have to deal with the freezing issues that you do. Hopefully one of the construction experts will chime in here to point out if it's a specific problem during a northern winter.

Janet