I ask mostly math questions and keep them open even after I've selected a best answer, just in case someone wants to comment or add a different solution. I once had a person sign up to Hubpages just so they could answer my question, so you never know.
Question askers shouldn't feel obligated to select a best answer, especially if it's an open-ended question, but I think people do like that reward and it may motivate more people to answer. If you had a reputation as someone who never selected a best answer, would fewer people visit your questions? I don't know.