It all depends on the type of pool you have, the landscaping in the area, and the water table. However your manufacturer’s warranty will not cover damage to the safety cover if your water is too low over the winter. For most brands this is no more than 12”-15” below the cover. In general concrete pools water level should be below the skimmer and tiles. Always check with the requirements of the pool structure to verify that the pool can be safely maintained at these levels.