I think [this blog post](http://blog.stackoverflow.com/2011/06/optimizing-for-pearls-not-sand/) by Jeff Atwood explains the original SE philosophy regarding voting best:

*... therefore, the only logical thing to do is to **maximize the happiness and enjoyment of answerers.** If this means aggressively downvoting or closing unworthy and uninteresting questions, so be it. Without a community of people willing to answer questions, it really doesn't matter if there are questions at all, does it?*

The only official help is this  help page regarding the
[vote-down privilege](http://serverfault.com/help/privileges/vote-down) which explains a bit: 

> **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.

--------

Personally I always like to point out that nobody complains about getting an up-vote and gaining some fake internet points without a extensive explanation.

I use the [AutoReviewComments](http://stackapps.com/questions/2116/autoreviewcomments-pro-forma-comments-for-se) action script with the following pro-forma text to explain downvotes on a Question:

> By philosophy and design votes are anonymous and **neither voting [up](http://$SITEURL$/help/privileges/vote-up) nor voting [down](http://$SITEURL$/help/privileges/vote-down) requires any mandatory explanation**. The tooltip that appears when your mouse pointer hoovers over the down button states: *"this question does not show any research effort; it is unclear or not useful"*. Also questions can attract a down vote when not [well written](http://meta.serverfault.com/a/3609/37681), not quite [on-topic](http://$SITEURL$/help/on-topic) or missing details.


I rarely use it but I also made a slightly different stock answer to explain downvotes on Answers:

> By philosophy and design votes are anonymous and **neither voting [up](http://serverfault.com/help/privileges/vote-up) nor voting [down](http://serverfault.com/help/privileges/vote-down) requires any mandatory explanation**. The tooltip that appears when your mouse pointer hoovers over the down button next to answers states: *"this answer is not useful"*  which may be subjective. The instruction is to vote down on any egregiously sloppy, no-effort-expended answer or one that is clearly and perhaps dangerously wrong. We also don't like spam or answers that  only contain links/code/commands.