My rule of thumb has been that if it was worth my time to read it (from a knowledge pov) then it's worth an up vote. I think this puts me on the generous side of voting. The voting guidelines suggest voting up questions and answers that are "especially useful", but I find I tend to vote for Q&A that are "somewhat useful". Though I've never come anywhere near the 30 vote/day cap.
What other rules of thumb should or do people use? For example
- Is it fair to say that if you accept an answer then you should vote it up?
- If you learned something useful from answering a question, then you should vote up the question? Even if it was badly written, or a "dumb" question.