Since your GMAT score is based on how many questions you answer, the best option is to make an educated guess on every question. This is partly to maximize your score and also to avoid getting stuck on a question so you don't answer the other questions. However, it depends on whether you're in a quantitative or verbal section. For verbal, it's better to put a good answer for some and leave the others blank if it means getting the ones you answer right. For quantitative, it doesn't seem to make much of a difference either way for your score. If you're trying to get a top score, it's best to answer the quantitative questions with your best guess. Hope that helps!