Depends on the question. If it's helping me make a shopping decision, I do - no sense people weighing in months after I've finished. If it's just asking for general opinions on a topic that will still be relevant in the future, then I leave it open. Also, you can still manually close the poll later if you want to. You can even go back and close old polls that were made before the forum change that added that feature. One of my 'help me choose' polls from September got bumped recently, and I was glad to see I could close it and let it die.