That's a very good question but I think every user having a privilege to delete may answer it a bit differently.
Amount of delete votes is limited and one better takes care to use them efficiently. From this perspective, questions that are to be auto-deleted are lower priority indeed. When voting on these, one needs to decide whether it is worth it to "help" system to have a question deleted sooner than it would happen automatically, or it makes better sense to vote something else.
One kind of "doomed" questions I prefer to vote delete (YMMV) is those with titles looking like broken windows. "Help with career as programmer", "recommend me tutorials", "problem: syntax error when adding 2 and 2", stuff like that.
- This is especially so when question title contains words that are blocked at Stack Overflow - "problem" and "help". Very presence of such questions sends a signal to folks blocked by quality filter over there that they can try their luck here. If it additionally happens that such question gets migrated to SO (losing negative score along the way), this is not just a signal, a royal road for one willing to use Programmers as drafting area for off-topic implementation questions (that's by the way why I never vote to migrate such questions - at the very least, I'd edit blocked words out of the title before doing so).
Another kind of "doomed" questions I tend to vote delete is ones that already have two delete votes. It sometimes happens mind you that I find it harmless and prefer to leave it hanging around until auto-deleted, but this happens very very rarely. Overwhelming majority looks like a terrible content deserving speedy deletion.
This is not surprising, if you think of it. Single delete vote may be emotional, but for two high-rep users to fall into a blind rage is really unlikely. (I sometimes even wonder why moderators don't use this simple heuristics to check for questions worthy of a fast removal).
Only thing I would recommend against is to be emotional when voting to delete. As you can see from above, this will most likely lead to your vote being wasted without making a difference. Try sticking with carefully calculated decisions, re-check delete votes tab in your profile to learn whether these work as you wanted to or not, adjust your criteria if needed.