Especially on MathOverflow, I ask much more than I answer. On some sites I haven't answered anything at all yet (mainly because of my subject skill level). Yet I feel this is unfair and I'm taking more than I'm giving, and that I'm not worthwhile to site. Am I right in this thinking, or not?
There can't be answers unless there are questions.
If you are asking good questions that are well-received by the community and result in good answers that will help others, then your contributions to the site are positive and valuable. Don't worry about it.
Asking questions is only a problem from people who ask terrible questions and/or questions that won't be useful to anyone else. Or ask their question then, when they get their answer, delete it. These people are wasting other peoples' time. They're the ones we don't want.