I thought I'd recently seen the auto-delete criteria posted here, but I can't find it now...
I fairly frequently downvote closed, downvoted questions just to be sure they get auto-deleted. Since I can never remember exactly what the rules are for the auto-deletion, if something is -1 with no activity, I'll often make it -2 just to be sure.
According to the FAQ post on Meta.SO about deletion, my habit of adding another -1 just to be sure a closed question gets deleted isn't necessary:
- The system will automatically delete unlocked, unanswered questions that have negative score after 30 days.
- The system will automatically delete unlocked, unanswered questions with score of zero (or one if the owner is deleted), fewer than 1.5 views per day on average, and fewer than two comments after a 365 days.
- The system will automatically delete any question (and its answers) or answer with a score less than 0 when its owner's account is deleted.
- The system will automatically delete questions migrated to other sites after 30 days.