You should base yourself on aggregate percentage distance from the statewide numbers. "Voting for the State winner" is a pretty useless definition if the county was won by 30 points while the State was won by 3.
But a county won by a R or D by 30 points won't have voted for the winner every election since 2000.
It's rare, but it can happen. For example, Nevada since 1960 is often cited as a bellwhether because it always went with the winning candidate except in 1976... Despite the fact that from 1964 to 1988 it was always massively more Republican than the country as a whole (but it just so happened that this lean was only decisive in 1976). Nevada is not a bellwether.
