Getting back to the talent trees, some observations: You actually have to take off-spec/alternate path talents due to the point requirements for higher tiers.
For example, Unholy must take Perma-Ghoul and Scourge Strike, otherwise they can't get 8 points in their spec tree so they can advance to Tier 5. The only way for a max level Unholy DK to not have both would thus be to not spend their talent points.