My question is simple - when should I cast delete votes?

I've recently reached enough rep on one of the sites to be able to cast delete votes. I am not sure when a question is supposed to be deleted as opposed to just remain closed. What criteria should I use when deciding whether or not to cast my delete vote?

EDIT: I have decided that until there will be a clearer consensus on this question, I better stay on the safe side and limit my deletions to obvious cases of noise or extreme cases. I'm still interested in a more clear-cut policy, though.