Obviously answers on old questions are still desirable, and presumably this would apply to informative comments as well. But what about clarification questions?
On the one hand, additional information may allow for an improved answer. On the other hand, the poster has probably long since stopped caring about the question, or even forgotten.
For questions, my gut says ask if there's no accepted answer, don't if there is. Personally, I'd welcome any comments/questions, but I've got only 1 question and 5 answers to be commented on. I can see how seasoned users might have a different perspective.