I'm not that much up on the Ordo Hereticus, but from perusing the Codex I'd say you probably don't really NEED ISTs, but they do have their uses. Their minimum squad size is 5 and you are not required to take a Veteran, plus they cost 1 point less per model and can take plasma guns in addition to meltas and the suchlike. Their hellguns are bloody awful though and the max is 10 per squad, so even at full strength their paltry numbers and somewhat measly stats are a big disadvantage if you should be daft enough to let them get assaulted. If you're stuck for the points then you could always throw a few IST squads in, but you'd have to be pretty badly stuck.
