In most cases, the judges do not vote based on the dance-off alone.

Examples:

Judi made a glaring mistake in her dance-off against Katie that all the judges ignored except for Anton.

As brilliant as Molly was, Tyler and Kym were both far better in their respective dance-offs against her. Again, Anton was the only judge who picked up on this.

As brilliant as Fleur was, she also made a glaring mistake in her dance-off against Will. Once again, Anton was the only judge who picked up on this.

It seems that Anton is the only judge who votes based on the dance-off and the dance-off alone. However, I did disagree with him saving Tom, Wynne and Chris (Tom was the better dancer but Toyah was far better in the dance-off).

Craig has often admitted himself to saving the better dancer and completely ignoring the dance-off.