Do I need to switch off A/C external breaker for winter?
There is no need to switch off the external breaker serving the outside condensing unit. I have never done so in 40 years at our house. I have never heard of anyone doing so, and I have never heard it recommended. In our installation the external box adjacent to the condensing unit contains a double pole switch, not a breaker, but I only shut this off to service the unit. However, my experience is in the relatively warm winters of Dallas, Texas.