I do test my water maybe once or twice in the winter, but it probably isn't necessary. I mostly do it just because I want to be out there messing with my pool. I have never had to actually add anything over the winter. As long as I start the winter with my ph at the high end of normal and my cl high as well, my pool is fine until spring. But, when the air temp begins to warm up, I begin early to test because I don't want to let an algae bloom get started. I am usually anxious for pool season by then anyways, so I am usually ready to start testing. I actually go ahead and hook up my equipment fairly early so i can start circulating when the water begins to warm up some even though it may be weeks before we actually swim. By using my solar heaters, we can usually begin swimming at the beginning of May.