There are no strict rules on downvoting. You can basically downvote for whatever reason you like including personal disagreement.
The main function of the vote buttons is separating good from bad content. If you don't like a question for any reason, it usually means that this question can still be improved. May it be because of bad formatting, false information or anything else that's negative.
Generally it's safe to follow these rules:
You should upvote if...
- You think the post is useful and informative for you and/or other people
- You think the post is well written and structured (without ignoring the content)
- You think post needs more attention but you don't want to use a bounty
- You find anything else that's good about the post.
You should downvote if...
- The post isn't informative, misleading or provides false information
- You disagree with the content of the post (usually with more subjective questions/answers).
- You feel like an Important part is missing.
- You see a post which should be flagged.
- You find anything else that's negative about the post
You can either edit or downvote if...
- The formatting, language or writing style of the post is bad.
Now as I've already mentioned: There are no strict rules on voting. Just keep in mind:
Read the post you're gonna vote on and focus on the content. If you disagree with an answer, you can downvote it. Even if it's a single word you disagree with. The reasons for downvoting a post differ from person to person. If you have a personal problem with a user, don't downvote. If you have a personal problem with a post, you can (but you don't have to) downvote.
On meta things work a bit differently. Here up/downvotes are usually meant to show that you agree/disagree with a post.