If you are referring to your state's withholding worksheet, you may want to check the rules applicable to your state.
In general, if you live and work in the same state, you do not usually put zero on your state income tax worksheet. The zero entry is typically used when you live in one state and work in another, and there is a reciprocity agreement to avoid double taxation. Additionally, you might use zero if you believe you will owe less than the standard withholding based on your filing status.