One reason I've used 0 in the past is to allow for multiple income sources (e.g., an employee at one company and a self-employed side project). The worksheet al

One reason I've used 0 in the past is to allow for multiple income sources (e.g., an employee at one company and a self-employed side project). The worksheet allows for negative offsets to be added into the final deduction count recorded. It avoids the need to deal with quarterlies if the withholding amounts are close enough.