Probabilistically speaking (i.e. if you don't know down to the second when a mob is going to die), it's almost never worth it to not to use an IDC combo on a fresh target because:

- There is more up-front damage; TTT doesn't pay off unless you actually get to Full Thrust.
- Putting Disembowel on a target is never ever bad for DPS, except if there is a better target to put Disembowel on.
- Chaos Thrust doesn't need to tick for very long to pay off vs. TTT and usually lets you disengage just as soon ("you're the OT's problem now, buh-bye!").
- You can finish the combo on another target, which is almost always good (there is the possibility of extreme Chaos Thrust clipping).

For similar reasons, it's preferable to begin an IDC combo on a mob that is about to die.