Question: My home’s A/C has an external circuit breaker. Do I need to switch it off for the winter?

Answer: 
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.
No, there's no benefit to it whatsoever. Also, on most split systems, the compressor is still in use when the heat is on. For split gas systems, they normally "steal" power from a leg on the compressor circuit to power the blower.