Working voltage of TVS (or any protection comp) must be a little over max DC voltage to avoid current leakage thru protection comp.

For de-rating you should use + 10-20%. This means that Vbr (breakdown voltage) value is important, it should be so much above normal DC voltage that you will never reach that in normal conditions.

5.6Vdc is usable (Vbr for that is ~6.5V or bit higher). With higher Vdc components (e.g. 7-10V) you will have less leakage.

BR,
-Gusse-