What not to
Since downvoting is a kind of censorship, it must be applied with care.

Do not downvote content that expresses a different opinion than your own. Its author has the same right to free speech that you have. Feel free to post your own opinion in a comment instead, or in a separate post.
Do not downvote content simply because you dislike its author. In particular, don't downvote a post just because its author downvoted one of yours. That's childish.
Remember that many (most?) steem users are not (english) native speakers. Don't downvote posts for bad spelling and/or grammar. We're not at school here.
Some content may be illegal in your jurisdiction. Remember that steem is a global network, and that the content in question may be legal elsewhere. IANAL, but downvoting such content may get you in trouble, because the downvote is proof that you have seen (and thereby downloaded) that content.