in reply to When to --
You shouldn't be too concerned about a few -- votes. Sometimes you might get it just because of the natural variation among people.
Something could get 100% positive responses only when no one actually read it. You're bound to have "outliers" or disagreement once a while in a free open community.
I think a reason why many people may choose not to explain their downvotes is to avoid escalating flaming war. It's easy to happen when people get defensive.
I would hesitate to establish voting guideline, let alone enforcing it, for the sake of diversity. The self-governing mechanism here seems to work fairly well so far.