Use your downvotes whenever you encounter an egregiously sloppy, no-effort-expended post, or an answer that is clearly and perhaps dangerously incorrect.
This is word-for-word exactly the same as the policy for the StackOverflow site.
Now, on SO this makes perfect sense, as we do not wish opinions in technical topics, it's all about facts, facts, facts.
What about IPS? I can think of very few possible answers to IPS questions that would be "clearly and perhaps dangerously incorrect". "Correctness" is not a good metric here; IPS issues are per definition mostly a subjective thing.
This example shows where someone finds some answer (involving creating fake accounts on dating website) strongly objectionable, and another commenter linked said guideline along with, roughly, "opinions do not allow for down-votes".
So, following this line of thought, we end up with downvoting simply because a post is "sloppy, no-effort-expended". Fine, this aspect doesn't concern me as it is obviously good to get such answers to the bottom.
But what if I find an answer which I feel strongly about (for example, an answer suggesting to deceive, lie, ...) and which otherwise is non-sloppy and displays suitable effort. Am I allowed to down-vote?
If the answer is "no" (as per our current official guideline), then... "duh".
If the answer is "yes", should we reflect that in the voting guideline? How could the quoted part of the guideline be improved?