Often I see questions with very few up votes have answers that get quite a bit of up voting. I have myself have read a question,found it interesting and then read a very good answer. Later when re reading I see that I up voted the answer but never bothered to vote on the question. Shouldn't we be up voting the question if we found an answer worthy of our approval?
In most cases, if the question has great answers, it's probably because it was in response to a great question. However, it's possible for low-quality questions to have high-quality answers, and that would be a case where you probably wouldn't want to upvote the question.
There's an intentionally small amount of guidance on when you should vote on posts, because it's often subjective. The primary bit of guidance that the site gives on reasons to upvote/downvote are the tooltips:
This question shows research effort; it is useful and clear
This question does not show any research effort; it is unclear or not useful
If you think a question meets one of these criteria, feel free to vote. Just make sure you're voting on the quality of the content, and not on the user (that would be voting fraud).