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

Answer: 
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.
No, you should not be doing that. It will have no advantage to switch it off. Many of the houses run for long time with external circuit breaker that is working along the HVAC system.