Keeping your tax withholding accurate is essential to avoid surprises when tax season arrives. One of the most important tools you have to control your tax withholding is the W-4 form. Updating your W-4 regularly helps you prevent underpaying taxes, which can lead to penalties and unexpected bills. This post explains why updating your W-4 matters, when to do it, and how it affects your paycheck and tax return. W-4 form filled out on a desk showing tax withholding details