DecimationFollow

Decimation activates by a Shadowbolt, Incinerate, or Soul Fire that hits a target when it is already at or below 35% of its health. It won't proc if your spell does damage that brings a target to or below 35% of its health. Also, if your spell misses or is absorbed then Decimate won't be triggered. Otherwise, there's no distinction between a boss and any other mob.

In a word: yes.

just a slight clarification, due to how blizzard rounds numbers, a mob that shows to be at 35% is actually anywhere between 35.0(infinite)1 and 36, so a SB or incinerate that hits the target at this point will not proc decimation, (same thing with the 4x multiplier on drain soul) therefore it is advised to wait till the mob displays health of 34% before expecting a proc