I've just seen a string of answered questions, with several answers per question and often multiple upvotes on some answers, yet the question remained at 0 votes.
If a question is good enough to be answered, surely it's also worthy of an upvote?
I tend to up vote questions I answer; as you suggest if it is worth answering it is worthy of an upvote.
If you feel the question adds value to the website, upvote it.
If you feel it could be improved, suggest an edit or make a comment and upvote it once the change has been made.
An upvote means roughly, "I want to see more content like this!"
There's no need to be stingy, as you can upvote many times in a given day. There's even a badge for upvoting questions.