I follow a heuristic which says that, for the purposes of making a decision, any statement that leads to the conclusion that your decision doesn't matter, should be considered false. If Omega is a figment of your imagination, then it doesn't matter how many boxes you take, so for purposes of deciding you can rule out that possibility. You only need to consider the possibility that you're hallucinating afterwards, when deciding whether to consult a psychologist and get an MRI.