Yes, the methanol does deter freezing, and the more you use the less chance to freeze up. It also evaporates quicker the more you use, so putting more in just means you'll need to add more sooner. The reservoirs are not sealed, so it happens.
My system uses so little fluid that I usually find it dries out long before I use it up. My system instructions said to use premixed windshield washer fluid for the perfect mixture of water/methanol.