You should generally increase your withholding if:
- you hold more than one job at a time or you and your spouse both have jobs (Step 2) or
- you have income from sources other than jobs or self-employment that is not subject to withholding (Step 4(a)).
If you do not make adjustments to your withholding for these situations, you will very likely owe additional tax when filing your tax return, and you may owe penalties. For income from sources other than jobs, you can pay estimated tax instead of having extra withholding.