If you think a question is generally of bad quality, should you downvote, flag, or both?
Basically, if the post is completely irrelevant, flag it, otherwise if it's just unhelpful or poorly written, consider downvoting it.
When should I vote down?
Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
You have a limited number of votes per day, and answer down-votes cost you a tiny bit of reputation on top of that; use them wisely.
When should I flag?
If anything happens on our site that makes you feel uncomfortable or that, in your opinion, clearly does not belong here, please flag it and bring it to our attention!